1. ...create a sidebar with some contents... At least some of these communities have empty sidebars.
2. Every community needs enough moderators. A single-mod community is not "enough" for a healthy community because things can blow up when you're asleep or away, even in a community that was previously inactive. If a community member reaches out to offer to join a single-mod team... that contact warrants a response from the existing mod. Not necessarily to immediately accept the offer, but at least to discuss the possibility of extra mod coverage.
3. It's just not at all true that if others aren't posting there's no moderation work that could be done. Mods of inactive communities can jumpstart them by soliciting feedback on proposed rules, advertising them elsewhere, making scheduled discussion posts, and more. Some of these things can be done by a "regular" community member as well, but if community members try to include mods in discussions about how best to promote the community and the mods ignore them... that's a sign that the community is abandoned.
4. If a mod is notified that they're their community is about to get reassigned and they don't respond... the community is definitely abandoned.

All of which is to say, there are lots of way to detect abandoned communities when post volume is low, and the process I highlighted is the standard way to request a takeover.