These are video codecs. Presumably an audio-only app (e.g. IP phone) would be compatible too, so that would explain using neither of the two video codecs. WebRTC also has datachannels that use neither audio nor video.
This was already-existing terminology in the IETF working-group. A "WebRTC-compatible" device is one that does not conform to the standard, but which can talk to something that does conform to the standard )in whatever limited way it supports). By definition no codec requirements can be placed on it. This was spelled out clearly in the original proposal: http://www.ietf.org/mail-archive/web/rtcweb/current/msg13432...
"Why can't I see you on you on my webrtc compatible desktop app?!"
Like an end-user is going to recognize the difference in terminology.