Does that end up meaning, in practice, that stable releases are never broken, but nightly might be, for a tier 2 target?
More or less, yes. It is guaranteed that tier 2 at least builds so they'll be a nightly available every day though it's possible they might have a serious bug. To be honest though that's always a risk for nightlies even with tier 1 targets. Tests don't catch every potential problem (even if they do catch a lot) which is why there is a beta period before a release.
Windows on ARM couldn't be tier 1 until recently as there weren't Windows ARM github runners. Now that there are I think it likely that it'll be promoted to tier 1.