That seems odd? Debian 12 Bullseye (oldstable) has fully supported i386 port. I would expect it to run reasonably well on late 32 bit era systems (Pentium4/AthlonXP)
But otherwise, yes, Debian 12 should work fine as you say. Not so long ago I installed it on an old Pentium M laptop I had lying around. Did take some tweaking, turned out that the wifi card didn't support WPA2/3 mixed mode which I had configured on my AP, so I had to downgrade security for the experiment. But video was hopeless, it couldn't even play 144p videos on youtube without stuttering. Maybe the video card (some Intel thing, used the i915 driver) didn't have HW decoding for whatever video encoder youtube uses nowadays (AV1?), or whatever.
The CPU will be struggling with most modern video formats including h.264.
Nowadays on an n270 CPU based netbook I use mpv and yt-dlp capped to 420p, even if I can play 720p@30FPS.