Everyone plays better defense than Trae, but Devonte, statistically, is actually just as bad as Trae is on defense.
What's funny is if you look at advanced stats, Trae's Defensive rating (estimated points allowed per 100 possessions) is 111.9. Graham's is 111.8. They both have a .4 on Defensive win share. Devonte is not a good defender, and that's not why he won the award. He won because he was on a winning team.
For reference, Jevon Carter, defensive player of the year in the Big 12 has a defensive rating of 103.3 and a defensive w/s of .9, far better than either of those two (obviously)