your OP doesn't appear to acknowledge that secular ethics has even been attempted

Yeah and when an astronomer talks about the earth he doesn't spend an hour acknowledging flat-earth theories.

This is a far more extreme claim, and one which all the expert consensus goes against

You're just fiddling with words. And there is no expert consensus because the polling is biased (philpapers survey is a horribly done survey), and even if there were it would mean absolutely nothing. It cracks me up that people constantly refer to this horribly done survey to back up their views. There weren't even close to enough philosophers surveyed, the response rate was abysmal, and the survey just flat-out excluded any non-secular philosophers. It's a meaningless circle-jerk survey.

Whether that is, in fact, what it is to be free is precisely the point of contention

No it isn't. Pull your head out of your ass bro. If freedom does not include the ability to do otherwise, you're just weaseling with words.