It is to me. I’m not claiming I can twist things so that YOU will be forced to analyze language the same way I do…
I’m perfectly happy to say that many declarative sentences in ordinary language are not to be taken as the sort of thing which evaluates to a boolean (one of {TRUE, FALSE}). And for me, “The Godfather is the best movie of all time”, or “Sherlock Holmes has blood type AB-”, or “50 is a big number”, or “You’re welcome”, or “I christen thee David”, or “There is a string of 100 consecutive 7s in the decimal expansion of pi”, or such things, in certain moods, are examples of sentences I would not normally want to analyze in terms of some artificially overformalized rules for assigning them a value among {TRUE, FALSE} [for different reasons, in the different examples]. Such rules will often have little to do with the use which those strings of words actually play in my speech-life.
Of course, someone else who wanted to straitjacket everything into such a Boolean form of analysis could nonetheless formalize such Booleanizing rules, and I might even care about those rules in other moods. And so I could never force anyone to stray from their perspective that everything is either TRUE or FALSE, should they want to formalize everything that way. But to me, that’s no different from claiming that every sentence evaluates to one of {RED, BLUE, GREEN}, or a number between 0 and 20, or what have you, by certain rules… I could make up rules which did so, but they’re not of any particular significance for the use to which I ordinarily put my language.
If, for you, a “proposition” means “something which evaluates to either TRUE or FALSE”, then, trivially, all “propositions” will evaluate to either TRUE or FALSE. But then, as I see it, there are a number of utterances in ordinary language which are not usefully analyzed as “propositions”, in this sense.
[I would not even want to say that, for every meaningful proposition P, there also a meaningful proposition NOT P. Just because there is some rules in our language for when we are warranted to claim P does not mean there are also some corresponding rules in our language for when we are warranted to claim NOT P. But this is an abstract point which I suspect there is little use in my harping on in this thread…]