They each had their own repressive, murderous regimes before the Germans invaded Russia, both of which thoroughly victimized their respective populations. That’s what was being referred to. BTW, they were also allied from August 23, 1939 until June 22, 1941.
Note that I disagree with efs111 about the efficiency of a crackdown. Neither Stalin nor Hitler had access to the internet posts of credit/debit card purchase records that the coming regime will...and Silicon Valley is already gleefully helping Big Brother.
Stalin and Hitler were violent dictators, At first they formed a pact together and Stalin supllied the Nazis until June of 1941.
When Hitler invaded and used massive genocide against the USSR, Stalin was ruthless enough to ravage half of Germany.
There was no relief for the population of either nation, but the Poles usually tried to get to Russian territory because the Nazis were always planning to kill or starve somebody. There were days the Russians were lazy about it.