Possible explanations for this:
1- He chose to pull up his shorts, cut out the liner, and/or wear a smaller size to avoid thigh rub, chafing, pain and blood. He has some big powerful thighs and those shorts are a problem. He decided to risk swinging free instead of unavoidable friction.
2- He chose to save face by not responding to the problem. Ignore the truth and master the big lie. Believe that it never happened and others will agree. There is no place to hide on the track especially in the final on national TV. Happened during qualifying too. Trying to correct the issue would only bring more attention to it.
3- He's a senior so those shorts might be really old and worn out, more so if he took them on a mission.
4- It was the plan all along. Only he knows why. Tired of having to mask up? But he got away with it.
Look at the grin on his face at the 7:54 mark of the youtube vid. Happy for PB, 4th OA, and...
The guy deserves major props for this.