I don't recall where, but I've heard that before in the past. Perhaps in the kind of slop that makes the rounds on LinkedIn.

There is sort of a good reason for it, in the past. Before the overhaul, Microsoft Speech used to skip Facebook posts, and read the alt text instead. It is now, however, more sane. Facebook was pretty darn bad at accessibility in its early days. A lot of intermingled broken spans for text, causing stuttering and other issues.

However, today, most reading systems prefer the "title" attribute, to the "alt" one. If title exists, it'll read that and skip alt. Some always skip alt, regardless of it exists or not.

Figure and figcaption are about the only way to get good and consistent behaviour, but you don't really control how those happen on most social media platforms. You throw everything you can at the wall, and see what happens. And it might change tomorrow.

Today, I'd say the above is bad advice. An image description is a good practice. Repeating yourself, isn't.