There's no clear answer I see there, and I agree with how you phrase the question: there's no way Stevens isn't aware of all this. There are a few theories, none of which are super convincing by themselves:
- Stevens knows the numbers, but doesn't trust them in the case, or thinks that Turner still has potential by dint of his pedigree, so he wants to keep playing him in hopes that it turns around. This the Stevens is trusting his gut theory essentially.
- Ainge is more or less forcing Turner on Stevens. I've seen no hint of this however, and it doesn't seem like something Ainge would do.
- The Celtics have even better numbers that show Turner is actually useful. I find this pretty unlikely. Public "all in one", metrics match propriety stuff pretty well from what I've seen, and Turner doesn't seem to have the kind of skillset that lends itself to this (unlike, say David Lee, where I could see him being underrated).
- The Celtics didn't really care that much about winning 40 vs. 43 games last year or something, and in fact may have preferred to win fewer. Playing Turner was a subtle tank. We're going to see Turner play surprisingly little this year under that theory, unless the Celtics are in lottery contention again.