Here's what I think happened.
Lyles was low key worried about the 200 ever since the US trials where he seemed to not be as effortlessly running as he normally would like (he went a little overboard on the 100m training)
He had to strain 110% in the 100 in paris. 200 comes around he runs a preliminary and knows he's getting beat by bednarek and probably someone else. His face was very telling then.
He felt a slight cold(not shocking given the travel) and proclaimed himself covid positive without taking a test. Going into the 200 final if he had won the race he would have told no one(hence his big entrance). Since he lost he could pull the covid excuse.
What's a shame is no one would have cared if he lost and got bronze. He's the 100 champ! And he had to give up the 4x1 and probably helped cost the team a gold there as well to keep the lie going.
Sadly it's my memory of him of the games. Wish I didn't have the image of him sitting in a wheelchair with a mask on...