Hi,
I have no other idea.
I actually tested the modification in my end and it works just fine.
I actually don't know of any other way to have a checkbox unchecked by default in HTML. That's how it is supposed to be done and the solution which works.
So if you don't see any difference, it means that you must have edited the view file for another template than the template you're using on your frontend. Or you've modified the file from your test website while you're looking at the checkout on your live website or vice-versa. Or we're not talking about the same checkout.
This modification I proposed is for when you create/edit an address in the checkout's "address" view. There is a similar checkbox on the registration/guest form of the "login" view but the login view already has a setting to uncheck the checkbox by default so no modification is necessary. Looking at your screenshot, I can't say which one you're talking about but since there is already a setting for the "login" one, I assumed you were talking about the "address" one, but maybe you don't know about the setting and are talking about the "login" one.