Yeah, I see this argument all over HN. And in a sense it's true that the affront here isn't against math per se, but against general purpose computing.
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.
I have always used the word "plaintext" to mean content that has not been subject to encryption or hashing, whether it is text, arbitrary bytes (including multimedia formats), or even abstract ideas which can be somehow subject to a cipher.
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.
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.