Nope, completely different. Xpra is resistant to network failures, while X server requires a stable network connection to keep the apps running. Xpra is also completely agnostic to the applications GUI: it just streams video and input events.

Rough X vs Xpra sketch:

                          |                    |
  display <-> X server    | <----X proto-----> | app
                          |                    |
                          |                    |
  display <-> Xpra client | <---h264/input---> | Xpra server <-X proto-> app
                          |                    |
                          |~~~~~~network~~~~~~~|