We have a language, a dictionary that drives what words mean, and the purpose of all that is to facilitate accurate useful communication. Deciding on your own what words mean is like deciding to speak in a foreign language and expecting non-speakers of that language to understand you.
You can make your own definitions for words but then you aren't communicating in English. At that point it's become another language that only you know and everyone is going to keep arguing with you becuse ite to the English language they'll always go with those definitions when trying to understand you.
I did not "make up" the definition any more than "narrativist" is a made up word. It comes from New Simulationism - Sam Sorensen.
On the other hand, I do think D&D is a simulation of a fantasy world, just a simplified and crude one. Fidelity and granularity will never define what a simulation is and does not with any definition of the word. For example we've had simulations for galaxy formations for decades and as we have more sophisticated tools they hopefully become more accurate. The models we use today are better (but still don't accurately match all observations) than the ones used in the early 1960s, but those early simulations were still simulations.