Ah, it looks like they've fixed that bug, then. It used to be that regardless of your autofill setting, basic auth would be presented; this was because the browser API requires basic auth to be non-interactive and an arbitrary decision has to be made.
From what I can see, this issue was till being reported in April[0] but perhaps it's been patched in the mean time. The devs were been going back and forth about this so long that I stopped paying attention to the issue after a while.
[0]: https://github.com/bitwarden/browser/issues/1408