"But what makes you affirm so confidently and emphatically that it’s not he?"
I will not conceal from you that Grigory himself confidently affirms and bears witness that you must have run from that door, though, of course, he did not see you do so with his own eyes, since he only noticed you first some distance away in the garden, running towards the fence.
There are no more uses of "affirm" in the book.
Show samples from other sources
Maria affirmed that she has complete faith in the other members of the team.
The artist has one function—to affirm and glorify life.