"But I've read somewhere else that you should do it differently!"

Sure you have. I don't deny that. I've read lots of alt-text and image description guides. You can find them here.

However, not a single one of these addresses the peculiarities of describing images in the Fediverse and fulfilling the image description and alt-text requirements defined and enforced by Mastodon's user community.

Very few of them even deal with Mastodon at all. What few do usually only explain how to enter an alt-text on Mastodon's Web interface, and they're done with it. They still assume that the same rules for alt-texts and image descriptions apply in the Fediverse as on professional websites.

This simply isn't true. And I'm the only one so far to ever make it part of an alt-text/image description guide.

To give you one example: Every alt-text guide out there tells you to keep alt-texts short and concise and no longer than 200 characters or 125 or even fewer. On Mastodon, in stark contrast, people write alt-texts that are several times longer. And they aren't scolded. They aren't criticised. In fact, alt-texts in the Fediverse that are complete no-nos by professional accessibility standards are being praised and featured in the Alt Text Hall of Fame. I'm not even kidding here.

Verdict: Yes, you've read it. But no, you shouldn't do it differently. Not here in the Fediverse.

On the open Web, you have to play by WCAG and WAI rules. In the Fediverse, you have to play by Mastodon community rules if you want to keep your reputation and reach intact. If you write an alt-text by WCAG 2.2 standards for the Fediverse, chances are that you'll be at least criticised by someone on Mastodon because your alt-text lacks details. If this happens repeatedly, chances are that you'll be sanctioned.