They are compression socks. a lot of the European racers I saw recently had them. Look at some photos of IM germany, they all wore them. They are supposed to help blood flow (if I’m not mistaken)
I did fail a bit though as a roving reporter. I stood with a few folks from Wisconsin and we saw how a camera bike took bumped into two racers during the run. One runner and the motorcycle went down, the other runner kind of just continued after stumbling. The speed was not big of the motorbike, and the runner who had fallen actually after shaking off the shock and surprise, shook hands with the motorcycle guy and the camera dude. He then went on back on the run course.
Ah! I am DEFINITELY looking forward to your photos newbz! Yours are usually better than any other ones I see, pro or otherwise (of course, you are basically a pro). Good to see you there, however briefly!