CD has had a bunch of injuries over the last year plus (and prior to that too...he's a bit snakebit) and I'm assuming that he is now or was recently still dealing with it. He specifically refers to a "dead leg" issue in his IG post after the Olympic marathon trials, in which he DNF'd. He also has a subsequent post where he refers to doing rehab.
I actually thought it was pretty cool that he ran. Given his results in the earlier meets, he HAD to know beforehand that he was going to get last, and quite possibly be far back like he was. And if he was hoping to pop a respectable race, I thought it was even cooler that he finished it out.
Not that I normally think completing a race is a big deal, but that's got to be a blow to the ego to be a guy of his credentials and be running way the hell off the pace from the get-go. I would think that most pros, and most runners in general, would take a hard pass on that. Says to me that he's a pretty secure dude and doesn't mind taking an a$$-kicking on the road back to fitness.