Yes, that 21.80 mph run by Zeke his rookie year (December 2016) was indeed super fast and his fastest ever as a Cowboy. Amazing considering he only ran a 4.47 during the NFL Combine in 2016. I doubt we will ever see Zeke run that fast ever again in a game. It's been over 3 calendar years since he accomplished that, and Zeke has only gotten bulkier since then.
As for Pollard, I believe he could top that mark if he gets more carries and is able to break a run out in the open for at least 40+ yards. That should give him enough distance to show what his top end speed in mph could be. I believe Tony Pollard could run faster since his best 40 time was during his pre-draft Pro Day where he officially clocked a 4.37 in the 40 yard dash.