Players have to develop strategies to create space. At every level you move up, it gets harder. A select few like Jaylen Brown can just blow by people even in the NBA, but the majority of players have to learn a better set of techniques at each level, and remap how they read and make plays, always having to do things a little quicker and better.
Pritchard is always going to be bottom 10% athletically, but he is crazy skilled, and he may work out how to play against elite players who are far more athletic and body him up. There have been a number of guys without athleticism who have figured this out even in the NBA over the years, but it ain't easy.
This takes reps. He wasn't asked to do this much last year. Mitchell has NBA all-defense potential. I wouldn't bet against him in the long run.