It doesn't change anything for either interpretation, since it still never explicitly references Con changes retroactively affecting hp. Arguably, what the "hp don't change" rule means is that, for example, your max hp doesn't become 29 if you change into a Ape.
How do we know that? Because this ability is inherited from alter self -- which doesn't affect a creatures Con score. So that's the only possible thing it means in the context of that spell. Polymorph inherits that and never states either way what to do with Con changes so it isn't clear.
The errata doesn't help this case at all, since it exists soley to bring the DMG inline with the already existing 3.5 PH version of polymorph (and the errata document makes that clear). And the DMG inherits the same ambiguous context of that rule.
There is no way to determine who is right or wrong in this case, since both sides continue to rely on ambiguous text. Both interpretations are equally valid, so the only thing to argue here is opinion.
IMC, Con changes due to polymorph do not affect hp totals. I don't care if that's a house rule or not. That's the way we play it.
