Hydroxynorketamine

They did some animal research that suggested that antagonism of the α7 nicotinic receptor wasn't responsible for the antidepressant effects. From the Wiki article: "[S]ubsequent research has found that dehydronorketamine, which is a potent and selective antagonist of the α7-nicotinic acetylcholine receptor similarly to hydroxynorketamine, is inactive in the forced swim test at doses up to 50 mg/kg in mice, and this is in contrast to ketamine and norketamine, which are effective at doses of 10 mg/kg and 50 mg/kg, respectively.[9]" This seems to be in line with the fact that we aren't aware of any other α7 nicotinic antagonists (e.g., memantine) having antidepressant effects like those of ketamine. You're right though, the action could be working or synergizing with something else, although I kind of doubt it myself. It's certainly a possibility though. We need more research.

Ketamine/HNK don't act directly on the AMPAR. They affect the AMPAR downstream via a transient increase in glutamate release (link). So it's a different mechanism, yeah. And we have pretty much no idea what, which is really interesting...