Does it only work with round seat posts?

This is sold, but no- the foundation stand work with all shapes of seatposts, up to 3" deep.

Full review and more pics here:
https://www.jensonusa.com/...nt-Bike-Repair-Stand

"Overall, for $50 you can't go wrong here. I mounted mine up high toward the ceiling and it has made working on my bike more comfortable and enjoyable. This extends 16inches from the wall and can support seat posts up to 3" wide. It fits my Allez Sprint just fine at ~2 inches and aero profile.
The mounting holes are roughly 2 1/2 inches apart.
The clamp is plastic and solid, the rest of the mount is steel."