if it it is obvious what a page is about, then the importance of the H tags become less and less.
This lines up with my experience in recent times, too. Rather than a formula that just adds various factors up, the algo seems more to look for relevance signals and then their reinforcements.
The way I think of it, the "seed" relevance signal could be one of many different things, on-site or off-site. Once the algorithm identifies that seed, it sort of learns how much the decision can be trusted by examining other areas for reinforcing or undermining signals.