I'm on your side. AC considers narrow-body J to be equivalent to PY in the context of Signature Class, as shown by how they marketed and sold the forward 7M8 cabin on intercontinental routes.

Given that there was no catering, what did you gain from being in J over PY?

Zone 1 boarding? Lounge access (which it sounds like you didn't even use)?

I'd try again with a very concise argument that the product difference between 788 PY and 321 J is virtually non-existent, and therefore, you want the bid amount back.