TL;DR: was Hawking a strong or weak atheist, did he think atheism was provable?
Something may be true but not be provable (theism or atheism depending upon one's point of view). There have been more than enough arguments about theories, but I'll say that I believe both the theories of relativity and evolution to be true. And even if one believes either or both have been proven, they were true before being proven, and true before they were even formulated as hypotheses. Or if one prefers laws to theories, gravity existed and had effect before anyone knew what it was, before life existed (I'm giving the atheist view here and/or seeing God or gods for the polytheistic, as not being life per se...

), before anyone formulated how it worked or why.
In quantum mechanics (uncertainty), mathematics (incompleteness), etc., rigid STEM fields, it's accepted that not everything can be known.
I have no problem with someone who takes the eminently reasonable position that there is no proof about various religious matters, nor who chooses to live their life that way (though I disagree...

). I do take issue with strong rather than weak atheists, however, those who claim that atheism is provably true. To me, that seems to be yet another form of unreasoning blind faith. And not that I think religion and science generally overlap, but it seems unscientific to claim such is provable. To claim it as
true that is, what one believes, may be reasonable, though I think agnosticism is more logical, but I'm coming from the perspective of a theist so have my own biases.