There is an incredible (alas unfinished) multipart Grothendieck biography by the German mathematician Winfried Scharlau: Wer ist Alexander Grothendieck? Anarchie, Mathematik, Spiritualität, Einsamkeit. I think an English translation exists, at least for the first and in my opinion most interesting volume: Anarchy. It mostly deals with Grothendieck’s childhood and his parents, who lived unbelievably fascinating lives as anarchists in pre-war Berlin.

The other article looks interesting, though it too has "blue banana" errors. Laurent Schwartz's autobiography "A Mathematician Grapples With His Century" has a more believable account of Schwartz and Dieudonné's work with Grothendieck. The Grothendieck Circle website (https://webusers.imj-prg.fr/~leila.schneps/mitanni/grothendi... since they haven't done a good job renewing domains) has a lot of Grothendieck's own writings (mostly in French) and Wilfried Scharlau's biography (in German). I tried to read those some years back but my language skills weren't up to it. Machine translation is much better now than it was then, so I might try the lazy approach.

Personally, just from the implausible level of first-person detail throughout I thought it's immediately clear the book is exceptionally well-researched but still technically qualifies as historical-fiction. We don't necessarily know what these people thought/ate/did minute by minute on the important days at that level.

But I think the biggest "sin" in terms of mixing fact/fiction was mostly implied and not actually stated. What's implied is that Groth saw inside mathematics some kind of terrible truth that motivated him to stop working and withdraw from the world. I don't think it's stated explicitly, but due to proximity with other topics in the book, reader is invited to conclude that there was a discovery of some kind inevitable doom, possibly a super weapon, etc.

We don't know that, but in a lot of ways it might be more surprising if he never thought along those lines. My understanding is that the other limited sources really do say he was talking to God in dreams, preoccupied with apocalyptic visions, became more interested in physics, politics, religion, the problem of evil, hostile entities ambiguously demonic, etc