Yeah, that demonstration was just to show that, despite how MP3Tag displays them, they use the same underlying field in the file. It doesn’t fix Picard in any way.
My guess is that rating fields are ignored because someone moved the primary display to the context menu (See the stars when you right click? Those map directly to the same underlying field we’re looking for.) but didn’t think about the case where the user/program they’re associated with is not the same one Picard uses to store them. It’s an understandable oversight – “Now that I’ve built a better display for the ratings, I can hide the raw data Picard stores. And since, for every other program I use, I either don’t rate songs or the ratings are stored separately from the files, Picard’s data is the only one I ever see.” – but as you’re saying, it is definitely something that’s missing.