There are a couple of places on the site which mention "all airlines" so I signed up. I can report that it will perform searches based on all airlines (or any that you specify). I'm not sure why they chose to only show AA on their screenshots. Everything seems to work well and I've been able to pull up seatmaps for the upcoming HP and UA flights that I have.

All of the information in the site is available free somewhere else, but it's REALLY nice to have it all in one place. The site is fast and I haven't had any issues with it. Worth $10/mo??? I don't know...

I'm assuming that the backend is SABRE (hence the AA bias in their screenshots).