No, wrong. You haven't addressed shit. You haven't presented any solid argument to justify calling someone wrong for using the word as it is intended to use. You simply don't like this usage of the word, but you also don't have any way to justify your point, simply because you're wrong, but you still wanna act like you're right, so all you do is to repeat "prescription versus description", in hope to convince people you're right because you know those two words, when you obviously aren't.
You talk about descriptism, but you don't really seem to understand how it actually works. Descriptivism doesn't mean you applying whatever meaning it pleases you more to a word. That's not what descriptivism is.
Now, the simple reason why your argument falls easily apart is because descriptivism is all about how a language is used by the people, rather than how rules intend it to be used. The thing is: people use the word fighting to describe any sort of physical or verbal confrontation. People do use the word fighting to refer to boxing. So we are not looking at a situation where the official definition of the word is not used by the majority of the population. IT IS! That's why your argument is invalid. It's as simple as that. If the dictionary tells you to use a word in a certain way, and pretty much everyone uses the word in that way, then there is no fucking reason to tell people they're wrong.
Now, let's say no one used the word fighting to refer to boxing and simple confrontations, and most people used it to refer to complete forms of combat where everything goes. IN THAT CASE your whole speech about descriptivism would make SOME sense. But even if it made SOME SENSE, which it doesn't, it still wouldn't give you the right to call people wrong for following the dictionary. Plenty of words have obscure meanings that most people don't know about, but that doesn't invalidate them. Just because a lot of people are ignorant the official rules of language don't become less valid. Descriptivism is more about giving new and more options than taking away those already established. In this sense, a word can also mean something that isn't even in the dic, but doesn't erase all the other meanings.
Oh, and btw, descriptivism has a lot more to do with rules of grammar. It's not about you creating your own vocabulary.