Actually it's at least the third - Undertone and Iron Lung (haven't seen either tbh) also did really well on v small budgets. And yes, it does seem like people (esp young people!) are really hungry for original, non-franchise horror movies to the extent they will actually pay money to see them in theaters. Backrooms is maybe a touch less impressive because it has a huge supply of producers etc backing it, but it's also doing remarkably well.

Obsession is actually doing better week-over-week (which never happens) and apparently is about to overtake the fucking Star Wars movie for daily business. So yeah I think it's a real shift.