Both Netscape Communicator and Opera did it as well.

That was my first thought when they mentioned how unique it was. But their claim of uniqueness is far from unique in the industry. Having grown up with computets in the '80s I have seen technologies come, the go, then come back again. Often it is because the technology of the time was not quite ready. In this case, integrated MUAs were dropped in the late '90s (if I recall correctly) because of a push for leaner and less cluttered software. The former was certainly a product of web browsers being considered as resource heavy for the time.