Stephen Kotkin's (he's very clear on his anti-communist stance so there is little bias in his view toward the Soviets) three part biography on Stalin goes into the Spanish Civil War. Stalin was neither as conniving nor as intelligent as his critics claimed.
Kotkin makes several observations that largely discredit parts of Orwell's claims. The three most important are that: The Republic would have collapsed without Soviet tanks which were invaluable to its early survival, Stalin did not want the republic to lose; a broad coalition was more likely to survive, and that the anarchist militias were already wilding before the NKVD started its notorious purges.
The civil war was a mess but not everything the Soviet's did during was done with active malice. There were also chain of command issues involved. But like the idea the Soviets stole the spanish gold when in reality it was more they did not recognize Franco's regime as a legitimate goverment, there is a real complexity to what was going on.