I wouldn't necessarily call it "security through obscurity" so much as just the nature of a web that isn't all in a few big baskets.

Besides, it's a knife that cuts both ways: the barriers to fluid movement means the worst people are kinda just stuck festering in a handful of places and everyone eventually learns where they are. Like, the big basket-style web has been a boon for fascists and their ilk in large part because there's lower barriers to entry and its possible to build a funnel from normal/mainstream boards to the more radicalized ones through intermediary communities.

But, when everyone knows, for instance, that something like Voat or Stormfront is where all the vitriolic racists are, there's kinda an upper limit to how easily they can lure people in since eventually they've gotta drag you there or else you'll probably slip away from the indoctrination, and that often means tipping their hands just a bit too soon to get past the "wait a moment, these guys are terrible people" filters.