Yes it is. Words have meaning, we would all benefit if we used them correctly.

I just did a search for "working theory" in all the dictionaries I could find. None of them had it. Where did you get this phrase from?

Yes, and it's not just usual ignorance, but they seem to be proud about their ignorance in a way that makes them behave irrationally and become offensive. For example how people downvote me in this thread, right now, although I got this treatment in many other venues, and in real life as well.

And of course the "argument" that language changes doesn't hold water. They are not merely using different words, they are de-facto eliminating the word that describes a "theory". People who actually make scientific hypothesis and scientific theories use the word correctly, it's the ignorants who can't use it, and who quite often will utter phrases like "X is just a theory!", like somehow that should lessen X.