That makes sense. You shouldn't normally be allowed to impersonate another username, which is exactly what a full import does.

Repository owner can edit comments arbitrarily, which already allows for impersonation, although to a lesser extent.

Also, you can impersonate commits freely.

I don't think allowing full import would be a big deal, especially if the UI made it clear that the comments were imported.

With full comment edit history that's no longer the case as much, although you can delete the diffs.

That's the one Github staff helped me use (though there were no public endpoints at the time). I recognize some of this. I don't think it's supported anymore.