I found this claim while searching about a great-uncle of mine, an Italian soldier who fought in the war (for the "wrong" side, unfortunately).
The regular troops were not all hardened professionals. Many were conscripts, and others labourers who had been tricked into coming to Spain, having been told that they were going to Africa as extras for the film Scipio in Africa. They might have numbers and technology, but this was not a force eager to be in the field.
It is said that the soldiers of the Littorio Division thought they were coming to Spain to star as extras in a blockbuster movie on Scipio in Africa!
I couldn't find the source for it, and nothing about its veracity. I believe it's likely to be a joke spread among the Republican forces, but I would like to be sure.
While I don't have enough knowledge of the primary source base to rule it out completely, I suspect your own conclusion is correct: a joke or exaggeration has been repeated by some credulous internet writers. I looked up the author of the first piece you linked, who aptly claims expertise in 'in writing about history and imaginary worlds', having studied medieval history in the past. This, nor the article you quoted, does not fill me with confidence in the claims.
There is a small but high-quality literature on the Italian forces in Spain, including a very recent piece by a well-known Spanish historian, Javier Rodrigo, who argues that these jokes and the wider Italian military reputation bely the large and important contribution made by the Italian contingent in Spain, as well as their often sincere belief in what they were doing as fascists and anti-Bolsheviks. That is not to say that they didn't include individuals with less 'idealistic' motivations, indeed they:
included adventurers such as 19-year-old Giuseppe Cordedda, without strong nationalist feelings, who set out for ‘unknown places’ in a ‘spirit of adventure’ and with no interest in politics or even the double pay he would receive. ‘I dreamed of new places, so many battles, acts of heroism and glory’. Some left for – and even returned from – a ‘tourist’s war’. Some enlisted entirely for work-related reasons, thinking they would be sent as volunteer workers to Ethiopia or to participate in the reconstruction of Spain. Others even expected to join agricultural projects, though such plans were undone by the Second World War (p. 5).
Such accounts may well be the genesis of the point made above - some were certianly in Spain unwittingly. Rodrigo's point, however, is that these were a minority, alongside others with atypical motivations. While it's not impossible that some thought there were going to work on a film set, anyone believing this was far from the typical case, and using such an anecdote to characterise the entire Italian contingent is seriously misleading.
Sources
Javier Rodrigo, 'A fascist warfare? Italian fascism and war experience in the Spanish Civil War (1936–39)', War in History (OnlineFirst).
Brian Sullivan, 'Fascist Italy's Military Involvement in the Spanish Civil War', Journal of Military History 59:4 (1995), 607-727.
Thank you u/crrpit, u/Georgy_K_Zhukov and u/Klesk_vs_Xaero! It was fascinating to follow your debate. I'm glad my question surprised you guys!