Yeah, alt-text is descriptive text you can add to images so that those screen-reader programs can read it, but IIRC it's also used if the image can't or won't load for whatever reason. You can even add it to nostr.build - at least through #Amethyst I have.

Regarding why the fuck they'd rant against it, I think it's just one more way to virtue signal.

"Yes, I #AATTMI (Add Alt-Text To My Images) and if you can't be bothered to add alt-text to a shitty Pepe meme you're sharing you're actively blindophobe and thus I'm better than you"

"Yeah even if its utility is marginal to the widest range of users and adds a 150% overhead to your meme-sharing workflow"