I understand that they're called fungus gnats because their piercings into leaves supposedly lead to fungus.

Frankly, I have never experienced that. I call those bark gnats, because they hover persistently around older medium, but I have to say, other than being eyesores, I cannot once think of a time that a plant or plants they've been hovering around caught some kind of rot. They may enjoy moist places to hang out in (and possibly breed), but they've never been a disease issue that I can think of or remember. Maybe I've just been lucky...