From the article:

These observations contradicted earlier hypotheses of strong winds or thick ice floating rocks off the surface. Instead, rocks move when large ice sheets a few millimeters thick floating in an ephemeral winter pond start to break up during sunny mornings.

A few points in your comment just didn't make sense to me, so I read through the article for clarification.

My first bit of confusion was that I'd hardly expect an ephemeral pond to rise to the level of halfway up these rocks on a wide flat expanse like this in a desert. That's a lot of water. Death Valley gets much less than 3 inches a year. Maybe it's possible, but the article makes no such claims.

Also, the idea that these rocks float seems at best dubious. Moreover if they did float, I find it hard to believe they would leave such prominent tracks in the clay. But they don't need to float. All you need is wet clay to reduce friction and a large ice sail; now you have movement exactly as the article described.