It's no big deal to bang shut a window from outside. So one lock falls into place inside.
Once this happens the window is officially secured. It may not be fort knox, but to the police on the night, it looked secure.
Agree he could have exited out of the open bedroom window. Although I don't believe he did.
It is quite a big deal with this kind of window for the catch to fall and make contact never mind to appear to be secure when banged shut.
Of course there is the question whether this particular catch was unusually loose etc. and it is impossible to know unless you had personally tried to do it.
It's impossible to know how he actually exited the property but it was accepted he had the means to enter and exit that night.