You are indeed correct, some artistic freedom is definitely expected from that kind of series. But relying on Russian propaganda sources and making Legasov a hero doesn't qualify as artistic freedom but misinformation. Also the representation of the soviet reality was at least inaccurate - my dad who was raised in the former soviet block summarised it as "representing how Americans think it was not how it truly was".

Chernobyl is a good and very interesting series and it's good that it raises at least some awareness about the catastrophe. But imo it could be more technically and historically accurate without losing its attractiveness.