> AFAIK: In lisp true means anything non-nil for most of the
> purposes.

Yes. And still, most predicates are defined as returning t/nil.

> This change I am not sure how convenient is it. It just ads
> some extra complexity to read with no benefit.

There are lots of places in the sources that already use (and ... t) to normalize the result to t. It's not a lot of complexity.

> Maybe it made more sense to fix the docstrings to say non-nil instead of
> forcing the code to return t?

That means that you're sure that there's no code out there depending on the (documented) result being t.