@peacememories
From reading the first section of the article, I believe that it is meant for people using screen readers, if you use a screen reader I doubt that you take the pronunciation at face value as well. And now instead of the Screen reader reading "bla bla IMAGE bla bla" it will offer an alt text.
But yeah obviously a way to differentiate between existing and locally generated ALT texts would be good, with the option to prompt for a new local alt text etc.
@mozilla