> when they leave their instructors behind and fly as pilot in command for the first time
This is very different. If an instructor is present the danger is clearly lower. The book deals with the phase where pilots first fly alone without external help.
Read the book and you'll see that the spike comes after the first few dozen flights without an instructor present (from 100 to 350 hours of flight time). I believe you're quoting the blurb, which doesn't reflect what's actually in the book.
This is very different. If an instructor is present the danger is clearly lower. The book deals with the phase where pilots first fly alone without external help.