The main issue is what the Soviet government did
before the disaster even happened. Someone, in the series it is implied it was a student, in Legasov's institution wrote a paper about what the risks was when AZ-5 was pressed. Apparently there was even a suggested redesign for the rods that would mitigate the risk (but it required lowering the maximum power output for the reactor, and thus required building more reactors, and so would increase the cost of the nuclear program by maybe 10-20%)
The Soviet government did something to shut that person up (and in the series Legasov implies he was part of that, I can't even find what he did exactly), repressed the knowledge (declaring it a state secret) ... and then a decade later Chernobyl exploded.
In other words: what happened is that the Soviet government refused to fix their nuclear reactors due to cost, and then that decision blew up Chernobyl, making tens to hundreds of thousands of victims.
Then, during the cleanup of the disaster, the KGB took additional measures to keep it hidden.
So yes it was oppression ... oppression is the cause of the disaster in the first place. And you can't forget that Legasov is not a hero: his career was built on oppression, not scientific accomplishment (there was a Soviet program to make sure Jewish students would fail at the institute. Legasov was the one implementing that). So of course Legasov can't be trusted.
Who knows, maybe the student who wrote that AZ-5 would blow up the reactor in the first place was one of the Jewish students whose career Legasov sabotaged.