There he joined the anarchist POUM militia... (page 309)Anarchist? Isn't the POUM rather famously Trotskyist (though I think some might even dispute that label)? I think it's name was something like the Unified Party of Marxist Workers (I'm not cheating by looking it up). It certainly wasn't anarchist though. The anarchists were the CNT-FAI.
I don't think this is trainspotter-ish. There were quite a few factions on the Left in Spain, but not that difficult to follow. In terms of orientation, size and influence this is a little bit like mixing up the Green Party and the Liberal Democrats today. What's more it's all spelt out very clearly in Homage to Catalonia. Maybe he hasn't read it - which would be odd as it's arguably Orwell's best book.