What you quoted is for the feature to add in images to PDFs. It doesn't work for existing PDFs with images already.
In the future, we want to be able to provide an alt text for any existing image in PDFs, except images which just contain text (it’s usually the case for PDFs containing scanned books).
That's how I read it atleaat. I could be wrong.
Yes you can use both but I've seen some front end developers blank out
altaltogether when they are usingfigcaption.I did not find this practice in MDN Web Docs but I found it in an other place:
I was just wondering what Mozilla's method was for finding these images and if they took other things in to consideration like decorative images.