This is really hard to agree with. There are some things text just isn’t good for, like sheet music or color theory. There are also many readers and authors for whom big chunks of text are hard to digest.
> Use illustrations as supplementary, not instead of information.
If anything, the author’s argument explicitly allows meme images and emoji.
As a physicist, yes, I wouldn't necessarily disagree that it is important to be able to distil concepts into language, but the language of mathematics happens to be very poorly supported by the computer-specific idea of 'plain text'. I have had too many times in the past when sending plain text emails meant filling emails with pseudo-latex equations and hoping that the recipients would be able to understand them, or needing to attach a PDF with the actual content. Translating the concepts entirely into a foreign language like English prose wouldn't help in understanding, it would just be ridiculous and inefficient. Similarly, if I'm showing experimental data, or any sort of graph, again, the language is not feasibly translatable into plain text. Markdown and well-formatted HTML and MathML emails with modern email clients allow me to communicate in the languages I actually need to write in, and express the concepts I actually need to express, in the forms they are most efficiently expressed in. Communications systems that support mathematics and images well are extremely useful for these other languages as well. As you point out, music has a similar problem: yes, it's probably reasonable to say that concepts should be able to expressed in language, but the language to express the concepts isn't supported by plain text.
And these different fields and languages do intersect, but that segment remains stubborn. For example, expecting all discussion of what a function in a maths library does to be entirely in plain text just to satisfy the affectations of a fanatical few would be unreasonable. Yet they do their best to impose. We see statements like these. We see services that might otherwise be useful needlessly enforce and evangelize their particular views on plain text. If anything, the fanatics become increasingly fanatical and unwilling to compromise: it becomes not just HTML and images that are decried, but any non-plaintext rendering of Markdown, or anything that isn't ASCII, or text that isn't hard-broken to 72 characters (or, in the case of this particular message, demands even on the particular style of English). It becomes not just a preference, but that non-plaintext messages should be attacked, or their authors aggressively educated on proper communication and etiquette, or non-plaintext messages be rejected entirely. I prefer chalkboards to whiteboards, but I am not going to storm out of a talk as soon as the speaker picks up a dry erase marker. Non intret Cato...
The rest of the world moves on, of course, and that segment likely wonder why they are disliked.