Ha ha, its not strong/weak sprint legs.
I’ve watched that repeatedly, where he sits down at almost exactly the 3:00 mark in the video. Still not seeing what you see, because he sat down at 120rpm and when I sit down at that rpm (or higher) it feels like what I’m seeing - bouncing until I bring pedal stroke under control, along with moving around on the saddle to get a good position. I’d prefer the Rollr movement to the nearly rigid non-movement of my Kickr, but understand if you don’t!
Like your MTB roots, around here there is a lot of degraded and broken pavement and a fair amount ‘skipping’ across bad road surfaces at speed. On every training ride.
Also I hate it when the bike jumps off the pavement, because sometimes I’m a flailing uncoordinated hot mess when launching! 