To be clear, it’s not the AA policy that’s relevant here, it’s the B6 policy, since OP is checking in with B6. The question is whether B6 would check the bag through (I have no idea if they will; I think that AS, for example, still would.) Once B6 checks it through, I’m pretty sure AA wouldn’t reject it. And your wording is correct: AA won’t do it; they can and in the past would have.

All moot anyway since the OP rebooked on a single ticket.