"You’d assume the lessening of antibiotics might be associated with improved health outcomes, especially since antibiotics are so overused."
It sounds more like something coming from Robert Kennedy, or one of those cranks who refuse to take antibiotics to treat strep throat, than from a mainstream researcher. Like, OF COURSE populations treated with antibiotics are going to do better in the period of a study like this. Under what plausible theory could you expect otherwise?
That's not to say that antiobiotics are an unmitigated good! I get that they have weird and complex downstream ramifications. It's just that those aren't the ramifications you'd expect to be able to measure from a study like this.