Unlike Google, Pinterest favours and promotes fresh content.
Instead of spending time correcting all your old pins, you should be spending time creating new pins that are 100% optimised from the beginning.
This takes longer but is certainly a more rewarding task to do. Instead of going back to update old pin descriptions, create and upload new ones.
However, if you have extra time and want to fix your old pins, feel free to add relevant keywords + hashtags in your pin descriptions based on the SEO learnings taught in the course. Fresh content always beats old content, but increasing your overall account quality, is never a bad idea.