Chat Control doesn't prohibit you from using whatever encryption you want, it mandates that your phone snitch on any plaintext that it has access to.
But ultimately, the right to general purpose computing (ie, the right to run a program that doesn't snitch the plaintext) is the same as the right to engage in the underlying mathematics.
I think it's a distinction without a meaningful difference.
CSAM isn't likely to be text that can be plaintext, is it? surely it would be image and/or video?
But these terms are certainly used different in different situations by different people. But yeah, images and/or video can still be "the plaintext" in the parlance to which I'm accustomed.