You said The fact of widespread theism is evidence for [theism is true]. This sounds mistaken to me. How do you derive it?

That seems accurate to me. Humans seem to have a bias towards induction and induction seems to work. Lots of humans thinking something seems to happen more for true things than for untrue things.

It isn't especially strong evidence. It is also evidence that you have probably already taken into account by the time you choose to ask "is theism true?" and should avoid double counting.