The Soviets weren't the ones conducting the studies I'm talking about. The Chernobyl incident was one of thr most studied industrial disasters of all time, and many Western organizations also studied the disasters and Drew up estimates of the impact. The 6,000 figure I cited above doesn't come from the Soviets Union, it comes from the World Health Organization.
If your reason for trusting on oral history from a private author is distrust incth Soviet Union, you still have to address why this book is more trustworthy than the multitude of of Western studies of the disaster.
Some of the people interviewed were experts, on the ground. Much of it talks about how much the "official story" omits. How the "authoritative" version isn't. Which was standard Soviet practice. Since you accuse the author (1) of "vested interests", well they cut both ways.
1) https://en.wikipedia.org/wiki/Svetlana_Alexievich