M. Gessen wrote a much better piece about the accuracy of HBO's Chernobyl: https://www.newyorker.com/news/our-columnists/what-hbos-cher... This piece seems a little confused, since Legasov wasn't the primary source for the show?
The show makes reference to "Midnight in Chernobyl" in its epilogue, I think it's safe to say it was one of the main sources of information for the show (though of course they took liberties because it was a historical drama).
I used as many sources as I could find. I was looking at research articles in scientific journals; I was looking at governmental reports; I was looking at books written by former Soviet scientists who were at Chernobyl; I was reading books by Western historians who had looked at Chernobyl. I watched documentaries; I read first-person documents.
And then there was Voices From Chernobyl, which is unique. What Svetlana Alexievich did there, I think, was capture an aspect of history we rarely see, which is the story of the people who you wouldn’t otherwise even know existed. We look at history from the point of view of the big movers, the big players, and she looks at history through the eyes of human beings. They’re all equal to her: Whether they are generals or party leaders or peasants, it doesn’t matter. And I thought that was just beautiful. It really inspired me.
So again this idea that anything not in the Legasov tapes was invented --- no. The show is a fictionalized retelling, but no, that criticism doesn't stand.