I don't remember this at all, but this might explain that. Or maybe a distribution like debian stable shipped with an outdated ALSA version, taken from the short period between release and dmix. Or just disabled dmix. Would love if someone remembered specifics.
I kinda assumed people mix up Alsa and OSS or don't remember anymore what actually did and what did not work before Pulseaudio was introduced.
In the early 00's (before PulseAudio), my desktop had an old SoundBlaster Live PCI card that was pretty common around the turn of the millennium. ALSA dmix Just Worked with that one.
Any other hardware I encountered required some kind of software mixing, IIRC. Not that my experience was extensive, but I got the impression that hardware or driver support for dmix wasn't that common.
Hardware mixing was killed, because it turned out that it is more efficient to mix several streams with CPU and move just a single one via the bus. It was also more flexible and without weird limits - for example, GUS could mix 14 streams at 44,1 kHz, and if you went above (up to 32), the frequency of each stream went down.
I kinda assumed people mix up Alsa and OSS or don't remember anymore what actually did and what did not work before Pulseaudio was introduced.