The knife he was gifted with his whittling book was a blue bone case stockman, so I'd imagine it was what he was using.
Weather the knife had a lock means nothing though , as it's the improper technique that caused the injury . I'll bet that were this injury caused by a failed locking mechanism the injury would be a lot worse.
As you say, and as I said, improper technique is to blame. Of course there is no need to learn proper technique through injury, especially when you have a book on the subject.
Again, it would be informative to hear what led up to it.