DT put up less stats than edge rushers and tackles are not an official statistic they are kept by the teams not the NFL.
13.5 sacks from a DT is incredible. For context Sapp and Randle only had more than 13.5 sacks once each in their careers.
Also Donald was a monster in the playoffs vs. Seattle until he tore a core muscle; Watt never touched Mayfield.
I watched several Rams games and Donald is an amazing player which is why he gets awards.