Ah, but then all you are doing is arguing over the definition of houserule. Do you see that? What you are saying, in effect, is that you are reserving that term for rulings that contradict the RAW ("At this table, all races start with 1 feat, and variant humans start with 2 feats" would be an example of a ruling that directly contradicts the RAW).
OTOH, you appear to believe that all adjudications that are not covered by the rules, or not expressly forbidden by the rules, are just rulings.
This is a fine thing to believe - but you're arguing about the definition of houserule.
As you see from the fireball discussion, people can muster arguments (support) for their beliefs about to worn objects on the target when a fireball hits. They can argue from common sense (of course a fireball would ignite objects on you!), they can argue from other rules (here's a rule about damaging objects!), they can invoke Latin (expressio unius means that rules imply that the objects worn wouldn't ignite), they can argue in pari materia (if you look at other spells, like acid splash, that should damage objects, they don't, so fireball shouldn't either), they can argue intent (the game is supposed to be fun/realistic), or whatever. But the point is - it's not in the rules. There's no right or wrong. There is just the rule that the house decides on.