Your probably right. I've never grabbed stills from Video so cant say what they are like and will probably do high speed stills as a primary method anyway.tbh it looks like an emergency replacement of high speed stills shooting for very very edge cases.
Grabbed jpegs from 8K stream may and will disappoint when compared to stills in raw format.
I cant wait to try the R5 frame rates as although I love the R for landscape & stationary things, it drives me nuts when I try and take images of fast moving subjects even in high speed AF servo. Anything other than single shot turns out with the focus not where I want it. Maybe I'm just rubbish at panning and following a subject but the AF tracking modes on the R5 will be a game changer for me if they are as good as they look. I cant wait to get my sticky little fingers on it.