Cannibalism was definitely reported and common. Normally if found to have occurred, and if the government official so desired, punishment was severe to include execution of the offenders. The thing is, while there were definitely punished instances of it, there were also plenty of cases where it wasn't. It's kinda hard though as it was so common that it wouldn't have been possible to prosecute everyone taking part. We even have records of local government officials hiding evidence of such acts, for fear of being punished themselves by higher-ups.

Human flesh was sold on the black market, and there are plenty of police reports on the topic of the consumption or sale of the meat. Quoting from Dikötter's Mao's Great Famine:

Human flesh, like everything else, was traded on the black market. A farmer who bartered a pair of shoes for a kilo of meat at the Zhangye [張掖] railway station found that the package contained a human nose and several ears. He decided to report the finding to the local Public Security Bureau. To escape detection, human flesh was sometimes mixed with dog meat when sold on the black market.

Basically, the practice of cannibalism itself was incredibly widespread.

So then on to the banquets. The relevant paragraph from Mao: The Untold Story is as follows:

The authorities staged “model demonstrations of killing” to show people how to apply maximum cruelty, and in some cases police supervised the killings. In the general atmosphere of fostered cruelty, cannibalism broke out in many parts of the province, the best-known being the county of Wuxuan [武宣], where a post-Mao official investigation (in 1983, promptly halted and its findings suppressed) produced a list of 76 names of victims. The practice of cannibalism usually started with the Maoist staple, “denunciation rallies.” Victims were slaughtered immediately afterwards, and choice parts of “their bodies—hearts, livers and sometimes penises—were excised, often before the victims were dead, and cooked on the spot to be eaten in what were called at the time “human flesh banquets.”

A similar story is recounted in Lin's Equity in the Chinese Law: Its Origin and Transformations

This mass mobilisation undoubtedly carried with it a certain element of coercion. The slogan "Whoever does not stand with us is against us!" carried with it not only a revolutionary enthusiasm, but a hidden oppressive message to divide the masses. Join us, or be our enemy! Such a black and white dichotomy suppressed the need for persuasion but more importantly, removed the freedom of choice. Emancipation, as assumed in such mobilisations, took such a totality that it allowed no alternative to challenge itself. The participants were more than spectators, but also involved in open, dramatic execution of the antirevolutionaries, where the masses shouted "kill, kill, kill" until their throats were sore and voices hoarse. Such extremism was further fuelled by vindictiveness from the peasantry. The cruel local tyrants were now avenged by the peasants through the same technique of the ‘human flesh banquet’, by which the victim was sliced, cooked and forced upon his family members. Such a revengeful bloody atmosphere was not much different from the traditional narrow-minded peasant vengeance, especially couched in local communities or clan patriotism. In general, such spontaneous cruelty augmented rather than defeated the communist revolutionary zest

This cites Lucien Bianco's Peasant Movements, which I imagine may be the source for the others, but they themselves likely rely on a well shared Chinese account. The specific banquet situation was written about in Chinese sources. We have accounts from a writer named Zheng Yi (鄭義) who wrote on it in a text titled Cannibalism Frenzy in Guangxi (廣西吃人狂潮), and who was there at the time. This is actually online in its entirety if you can read Chinese.

Zheng's account of the feast is as follows, for those interested:

南宁附近一山区小县，人口少，但杀人按比例全区第一。⋯⋯席间谈起上林文革杀人情况，一阵“阶级斗争的十二级台风”刮来，人们便开始乱抓乱打乱杀。一时间杀人如麻（数字不在手边），县城电线杆上皆挂满人头。问及吃人，满座皆称上林吃人不多，远远比不上武宣等县。人们记得的案例是：某将人活活剖腹取肝后得意洋洋提回家吃。半途见一人，问：被杀者同意你吃他肝了吗？答曰没问。不行不行，他若不答应，你吃他的肝没用。（本地迷信，吃甚补甚。）某将肝丢弃，又去抓了一个“阶级敌人”，用种种酷刑，逼迫被害者同意被食，遂活剖取肝而食。采访之余，老莫还向我谈了他死里逃生的经历：一日晨，被专政的“牛鬼蛇神”们正在田里劳改，忽一队武装民兵来押人去开批斗会。老莫见难友们都被押走，却无人叫他，便恭敬地问一民兵。那民兵考虑一番，说：“只叫我来带自己村的人，你去不去我不管。”老莫遂未去会场。而那日参加批斗会的人，全被打死，无一幸免……。

I'm a rubbish translator, but basically this is what you accounted in your original post. There was a frenzy of class struggle, what he called a "typhoon", and while the area had a small population, it had an exceptionally high murder rate. Then the rest is basically what you've already said and what was accounted in Mao: The Unknown Story. The whole text by Zheng is here.

So here's the thing. Did this happen? Yeah most likely. We don't have good reason to doubt the author. Cannibalism was super common, super reported even for all the times it wasn't reported (which we know because we have accounts of people's reluctance to do so). Did local government people do bad stuff? Yup, also widely attested. There's a saying in Chinese: 天高皇帝遠, the mountains are high and the emperor is far away. Basically, what happens far from the central government isn't really always something the central government can do much about, even were they to disapprove.

Was this sort of "banquet" super widespread? Not likely. This is the only account I can dig up of it happening just like this. So while it probably did happen that people were forced into cannibalism as punishment beyond this one occurrence, we don't have too much reason to think it was a regular thing either. Of course, basic cannibalism was more of a regular thing, and again, local government people could be super cruel and the Red Guard could be super super cruel, so, yeah.

Anyway, cannibalism was common, was often recorded as having happened in Guangxi where this story comes from, but not in any way limited to Guangxi, let alone a single reason or class, and given the other things that occurred during the Great Leap Forward.

In Mao's Great Famine, some specific police records are given, to give you an idea of the kind of situation you'd usually see. One is as follows:

Date: 25 February 1960.
Location: Hongtai Commune, Yaohejia Village.
Name of Culprit: Yang Zhongsheng.
Status: Poor Farmer.
Number of People Involved: 1.
Name of Victim: Yang Ershun.
Relationship with Culprit: Younger Brother.
Number of People Involved: 1.
Manner of Crime: Killed and Eaten.
Reason: Livelihood Issues.

While some people were certainly killed for the purposes of cannibalism, it was more common that people were just eating or selling meat from already deceased people. Many of the accounts are of people disinterring corpses rather than murdering people. Guangxi stands out as something of an exception, of course, but the local officials weren't exactly helping.

I think it's really important to try to keep in mind the absolute atrocity that the Great Leap Forward was for most of China's population. People's actions were unquestionably horrifying, but on all sides. A lot of people who were engaging in cannibalism really didn't have any easy outs here.

References:

• Cheng's Mao. The Unknown Story which you brought up

• Dikötter's Mao: The Untold Story, quoted above. Has a whole chapter on cannibalism.

• Lin's Equity in the Chinese Eaw: Its Origin and Transformations, quoted above

• Zheng's 《廣西吃人狂潮》, quoted above.

See also:

• Becker's Hungry Ghosts: Mao's Secret Famine. Also has a whole chapter on cannibalism during this period.

• Song's 《廣西文革中的吃人狂潮》, which also gives Zheng Yi's account and some more context

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edit: Someone replied about Dikotter sensationalising things. I can't find that comment now, either because it was deleted or removed perhaps. It was to the effect that at times he seemed to go out of his way to make things sound bad. I think that's completely fair, and I myself agree: He reads as though his goal were to make the PRC look bad. So, whoever left that reply: I agree on that point. I still think much of the points made in the text are still valid, despite not always agreeing with how they're being made.