I would have some doubts about the truth of a Stalin biography published in 1940.
The idea that Stalin got enlightened by reading Darwin has at least two big problems.
1) Where did he get the book? Remember he was in an Orthodox seminary at the time. I don't know how available a Russian or Georgian translation of Darwin would be in Russia at the time.
2) I don't think Stalin was smart enough to understand Darwin. If he did, then how could he have embraced Lysenko and sent Vavilov to the Gulag, actions which seriously hurt Soviet agriculture.
In fact, the whole thing strikes me as the Stalinist equivalent of George Washington and the cherry tree.
Would you have any doubt that Stalin endorsed what was in a biography published in 1940 in Moscow?