See a summary of Preston's view of Orwell's depiction of the POUM in Preston's guardian piece, " George Orwell’s Spanish civil war memoir is a classic, but is it bad history?" The main area where they conflict is Preston says at one point hundreds of POUM soldiers left the Aragon front in protest to new Spanish Republic military policies. He says they went back to Barcelona and held 60,000 weapons from the Republic, refusing to give the weapons up or fight themselves. Orwell directly contradicts this 60,000 weapons story in "Homage." He argues it's simply communist propaganda, and the POUM in Barcelona more likely commanded not even 1,000 weapons.
I think their views are somewhat reconcilable, as it seems the POUM did much more to help rather than hurt the war effort, as they had thousands of troops protecting the Aragon front throughout the war, and only a small portion left (Preston says hundreds left). At the same time, the POUM was pro-revolution, according to Orwell and Preston, while the majority of the Spanish Republic wasn't, so if the Fascists were defeated, there may have afterward been a conflict between the POUM and the Spanish Republic.
This is a tricky one to answer completely, as I've never done my own primary research into the May Days and don't feel comfortable evaluating the merits of specific claims regarding weapons and so on. The international and domestic political situation surrounding the May Day clashes is immensely complicated, and untangling who was trying to achieve what, and by which means, is very difficult, lending itself to very different interpretations depending on ones framework or political perspective. By way of demonstrating the sheer difficulty of getting to grips with these complexities, one attempt to chart out the political context (by one of Preston's friends, Helen Graham) resulted in what is still the longest scholarly journal article I've ever read, coming in at nearly 60 pages (the full reference is Helen Graham, "'Against the State': A Genealogy of the Barcelona May Days (1937)", European History Quarterly 29:4 (1999), pp. 485-542).
On the wider question of Orwell's narrative about 'betrayal' of the revolution by the communists, there are quite a few existing answers on the subreddit, including my own answer here, or an earlier one from u/Domini_canes. I've also written a post discussing Preston's particular dislike of Orwell, making reference to the Guardian article you mentioned. There is of course much more to say (60 pages worth...), and I'm happy to try and address any follow up questions you might have.