I wouldn't advise trying to break a side window in a car via a locking blade with the blade open. Your hand could slide down over the blade and you could wind up with a nasty gash if you don't manage to break the window in the first shot.

I'm sure he means strong in the sense that while closed the whole knife can take the impact of the window and still function?

I mean I understand you want the most pressure at the smallest point to break the tempered glass, but it sounds pretty dangerous with the knife open. I'd try with a file or something else before the blade.