In a sense the power of a dictionary is that you are allowed to use those meanings (indeed they mostly contain positive information and rarely what words do not mean)
> If you description was actually accurate, there would be no new slang (e.g. yeet)
Completeness ad accuracy are different.
> words would not change their usage (e.g. "they" is now also a gender neutral singular)
Speech can be sometimes accurate and sometimes less accurate. As a medium the value of speech is what you can express with it, it is in general not a form of art per se.
>> In one case I am declaring that something needs to be true, in the other I am declaring that I believe something as true.
>Those are not as different as you seem to believe.
I indeed believe they are quite different, 2=2 must be true in terms of the statements I understand it to be. Evolution on the other hand is something that I simply believe.
I cannot even fathom[1] what a proof of "not 2=2" could be, as in even if you had one I would be unable to understand it or believe it.
Evolution is something that instead can be disproven, even more than that a huge chunk of why scientist believe it is because experimental result could disprove it but instead keep confirming it.
[1] this is an important logical concept: for a statement of facts to be (at the very least) well formed you must be able to understand what it would be required for a proof and/or a confutation.