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.
There is zero extrapolation there. The rule is 1 hit point per day of rest. You engage that rule each day.
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.
This is not a house rule, because the rules for interacting and damaging objects are on page 185. The rule is that the DM decides the AC, hit points and any resistances for the object. The sword does the damage. No extrapolation is necessary at all.
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.
If your ruling alters or adds to a mechanic, it's a house rule. It does not apply to any other house but yours.
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 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.
Okay. Aside from the fact that there are rules for swords cutting cord, I have to say... And? Yes, the DM has to come up with stuff. NONE of those things are RAW. It's impossible for it to be. RAW is only what is explicitly written.
That doesn't mean that these are house rules.
It does when they are mechanical in nature.
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!
I bolded the problem with your statement. Unless they always ignite things or never ignite things, the DM has to make RULINGS, which are then rules for his game, since under the same circumstances in the future, the rule repeats itself.
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.
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.
Your extrapolation is a RULING, which makes it a rule for your game. Under the same circumstances, your rule will repeat itself. Were your ruling part of RAW, I would be required to use it unless I house ruled it away.