The "neither did" response to the claim is not that "neither the Communists or the KMT ever fought against Japan" but that "neither Chinese subset of people affiliating with Communists or the KMT counts as the 'real' superset which fought against Japan, which was the Chinese people as a totality."

However, that's mostly a pedantic argument over what counts as the "most real" grouping of a people. It's like saying "democrats and republicans don't exist, only people exist" which can be argued as true, but realistically is a pedantic statement.

The short answer is no. After the war and civil war, both the KMT and the CCP minimized the role of their opponents and maximized the scale of their own contributions in the struggle against the Japanese. The wartime fight against fascist imperialist became a prologue for Chinese Communism's eventual truimph in the CCP's history. In Taiwan, heroic Chinese resistance is undermined by a CCP preparing to stab "China" in the back, thus explaining the defeat in the Civil War. Neither historical narrative stands up to close scrutiny. The top comment when I accessed the thread (the Chinese people fought them") reflects a further strand in this popular memory that has developed in the twenty-first century that seeks to retain the nationalist narrative of heroic resistance but also depoliticizes the war. The actors may have change, but the script remains.

The grain of truth in these comments is that neither side was as effective in the fight against Japan as they claimed to be. For example, the Japanese 1944 Ichi-Go offensive was able to gain substantial ground in southern China at a time when Japanese forces were in retreat pretty much everywhere else. It's also quite hard to quantify the successes of guerrilla or irregular warfare.

Edit: It should be emphasized that there is an ongoing reevaluation and reassessment of Chiang Kai-shek and the KMT's military efforts against Japan. While the KMT's armies were seldom successful, previous generations of historians have taken this failure as evidence that the KMT was terminally corrupt and incompetent (see Barbara Tuchman's Stilwell and the American Experience in China for an example of this scholarship). However, the KMT was often in a very difficult situation throughout the entire conflict; having to contend with indifferent or ignorant allies, a massive refugee situation, and the Japanese themselves. Furthermore, the r/China thread also brings up the idea that most Chinese were indifferent to the war and who was in power, but indifference does not necessarily mean acceptance either. Even though elites tried to instrumentalize memory into a KMT or CCP nationalist struggle, the fact that the nationalist narrative has become a mass popular memory in both Taiwan and China suggests that there is some basic acceptance of this narrative among the populace and is not just an cynical manipulation of history to suit the needs of those in power.

Sources

He, Yinan. "Remembering and forgetting the war: Elite mythmaking, mass reaction, and Sino-Japanese relations, 1950–2006." History & Memory 19, no. 2 (2007): 43-74.

Mitter, Rana. Forgotten Ally: China's World War II, 1937-1945. [S.l.]: Mariner Books, 2014.