Riders are not permanently affixed to a bike, so I'm struggling to see how a tuned mass damper could actually be tuned in this case. The very act of lifting off the saddle would briefly change the required tuning, and it's illogical to tune to just the bike's resonant frequency when the rider would change that value.
If this were touted as a simple "mass damper", that might be alright. Though static mass dampers -- and tuned mass dampers that are improperly tuned -- can exacerbate oscillations under the wrong circumstances.
For more about tuned mass dampers, Practical Engineering just released a video on Nebula available now (and free on YT soon) about liquid mass dampers, which is a type of tuned mass damper.