You can't extrapolate a single thing and still be playing RAW. The instant you do, you have left WRITTEN and engages in a rule that applies only to your house.
There are so many counterexamples to this that it's hard to know where to start.
Gygax's DMG says that a PC regains 1 hp per day of rest. My PC rests for 4 days, so I extrapolate that my PC regains 4 hp. That is not a house rule.
The rulebook lists a sword as a weapon. I equip my PC with a sword. Later on in the game, my PC needs to cut some cord. I tell my GM (as my character) that I use my sword to cut the cord - the GM says "Yep, fine". That is an extrapolation from the description of the weapon I purchases as a
sword to the fact that it has a cutting edge that can be used to cut things. It is not a house rule.
In general, every application of a rule to derive a concrete consequences is an extrapolation, especially when (as in D&D) those rules are stated in non-formal, natural language terms.
I ask again: are you
really saying that it is a house rule to allow that a jug, purchased by a PC from the Basic PDF equipment list, is able to hold fluids? Or will spill if tipped over?
There is no rule that says fireballs, lit torches, burning oil or any object of fire starts things burning, unless the specific rules on that object say so.
There is no rule that says that jugs hold water, or that sword blades can cut cord, or that banging swords on shields makes a noise, or that characters with lips can whistle.
That doesn't mean that these are house rules.
The point of using natural language terms to describe game elements - fire, torch, burning oil, sword, jug, lip, etc - is so that all this information is conveyed without needing to be expressly spelled out. This is the key difference, too, between a RPG and a boardgame (or CCG etc). The fact that a fireball is a fireball doing fire damage and not a lightning bolt doing lightning damage isn't simply fluff text, like the italics on a MtG card. Nor is it simply a descriptor that triggers some mechanical interactions, like the keyword
flying on a MtG card. It tells us something about what is happening in the shared fiction, which brings with it various self-evident potentialities - such as the potential to ignite combustible materials. (Likewise the blade of the sword has the potential to cut cord; the jug has the potential to hold water; etc.)
the rules don't cover every circumstance.
But they cover some. For instance, the rules around fire damage cover the circumstances of whether or not sources of fire damage can ignite combustible materials, because they tell us that fire damage (read that again -
fire damage) is the sort of thing that results from burning oil, flaming torches, alchemist's fire, dragon breath, and conjured flames. These are all things that are prone to ignite combustible materials!
It's as certain that a fireball may set things alight as that a jug can be filled with water. If one is a house rule than so is the other; but I think it's obvious that neither is.
This isn't 4e, where the mechanics were 100% carved out from the fluff.
I'm not 100% sure what you're intending to convey by this, but 4e's mechanics engage the fiction at many of the same places as 5e's. In both games, for instance, the mechanics and rules generate exactly the same sort of reason to think that a fireball spell might set combustible material alight.
Your extrapolation has no bearing on my game and I am not required to rule against your extrapolation if I don't approve.
I don't understand the second clause.
As for the first: all you are saying that is that one person's application of the rules is irrelevant to another person's game. This is true, but has no bearing on the nature or source of the rules that they are applying. All it means is that I am not the boss of your game.