The original context of this discussion is whether these things are meaningful. It should be pretty obvious that the whole discussion presupposes that they are, including your own remarks. So since this is obvious, there is no need for further discussion of whether they are true or false in particular.
Huh? “Bitter” is an adjective—as you youself say, the noun is “bitterness”.
In both phrases—purple is (not) bitter and this bark is not bitter—it’s an adjective.
By the way, consider another phrase: Red is hot.
Is it true or false?
The original context of this discussion is whether these things are meaningful. It should be pretty obvious that the whole discussion presupposes that they are, including your own remarks. So since this is obvious, there is no need for further discussion of whether they are true or false in particular.