Interesting. It also made me look at the MDN docs again. img alt is consistent to that. I wasn't aware of the empty for omittable images.
I also looked at figure again, and in my interpretation it does declare that figcaption is to be used.
figure represents self-contained content. figcaption provides the accessible name for the parent. The accessible name is is the text associated with an HTML element that provides users of assistive technology with a label for the element.
The resolution order being aria-labelledby, aria-label, input[type=button][value], input[type=image]|img|area[alt], …