This has worked reliably in ALSA before it has worked reliably in PulseAudio (and, in fact, way before PulseAudio received any meaningful adoption, when Fedora enrolled everyone in PA's beta testing).
Well, maybe in the later days, but there was a long time when it didn't work well at all. I remember having to mess with the ALSA configuration to make it work. And then you also had apps which only supported OSS and getting them to work together with ALSA was an additional pain.
The reason for that was that while dmix was available for a long time, it was not enabled automatically on hardware without hardware mixing. More recent versions will silently (heh) enable it on hardware thats known to not have hardware mixing.
What later days? Software mixing worked fine in ALSA in 2004-2005, and OSS emulation had been working reliably way before that. The first PulseAudio release was in 2004, and it wasn't adopted by Fedora until way, way later.