You're right that it's more than "worldbuilding", and I was being unfair to that position. But I also disagree with your interpretation of the concept of a diagetic framework of a world created through interaction being absolutely essential to the creation of a TTRPG; or rather, that a TTRPG's world does not have to be entirely created through that interaction in order to qualify. As has been pointed out, you're tossing a lot of babies out with that bathwater, and leaving out a lot of middle ground that is easier to argue about that MMORPGs.
That said, participants in a role-playing server on a MMORPG are still engaging in the activity of creating parts of their world through their interaction and play. They're not creating all of it, sure, but you're also not creating the entire world of Faerun from scratch every time you start a Forgotten Realms campaign. There's a sliding scale, and anywhere you point that point is going to be arbitrary. Which, sure, is rather the point of a personal definition, but then that's a different beast from arguing another's personal definition is wrong.
Again, this isn't a personal definition. It's a definition I appropriated from academic work on the subject. Moreover, it's ...
an actual definition.
The problem with definitions is that ... they define things. If you wish, you are welcome to present your own, as I mentioned. I think you will see that once you do so, it is very difficult to create one that-
A. Includes all the things that you know to be a (TT)RPG; and
B. Excludes things that you know aren't a (TT)RPG; and
C. Doesn't exclude things that you think are a (TT)RPG; and
D. Also manages to, relatively succinctly, capture the essence of what a (TT)RPG is.
For example, my proffered academic definition, which is, again, for TTRPGs, most assuredly captures things that others would argue strenuously against- LARPs. Freeform. GM-less games that operate through other narrative authority (Fiasco!).
This doesn't make it either right or wrong, but it does provide a useful starting point. I don't think that the fact that it happens to exclude CRPGs, which are a well-known, and usually considered separate, subgenre of RPGs, is a fault of the definition. But if you disagree,
come up with your own, instead of insisting that this one is wrong.
It's a lot harder to create it, than to criticize others.