It might probably help you to look at the text of the "not an answer" flag reason:
This was posted as an answer, but it does not attempt to answer the question. It should possibly be an edit, a comment, another question, or deleted altogether.
This is IMO inappropriate for these answers because they do attempt to answer the question. The only place they fall down is that they don't meet your criteria of textual evidence, but otherwise they are honest and in-good-faith attempts at providing an answer.
Of the four options given for dealing "not an answer", none of them are really appropriate either. The appropriate thing to do is for them to be edited to add the textual evidence you're looking for.
So that establishes that "not an answer" is not the appropriate way to deal with this. Adding a comment requesting such evidence is good, and you can always downvote the answer if you feel it's not helpful to you.
However it's worth noting that regarding textual evidence on this SE, and particularly with regard to a work that the author has not yet completed (cases also exist where an author has died before completing a work), the hard reality is that sometimes this evidence just does not exist. In cases like that you just have to accept that an answer without textual evidence is the best you're going to get.