To be picky, these weren't armed rebel groups (at least not until the May Days). They were armed groups defending the legitimate government of Spain from a full-on mutiny and attempted coup by the armed forces and associated right-wing groups.
It was the middle of an insurgency, but these groups were fighting to suppress it, not support it.
Orwell was in a roughly similar role to one of the idealistic Westerners going to fight in Ukraine today, except the anti-Fascist side was much more politically fragmented, less well armed, and ultimately lost, leaving Spain as officially Fascist until 1977.
It was the middle of an insurgency, but these groups were fighting to suppress it, not support it.