I might be wrong, but I think that the RetroPie page that you link to, could be out of date.
Although, admittedly, there seems to be conflicting reports and information.
Apparently 32-bit support was dropped a couple of years ago, although that seems to have been for Windows only. From Obituary for 32-bit:
32-bit ARM devices are not affected by this decision and will continue to be supported.
There is a thread, dolphin install, where someone has the same issue as you are experiencing on ARM and apparently you need to do a custom install to make it work, although what custom steps are actually required isn't specified:
dolphin isn't available on standard builds of retropie due to its 64 bit requirements. Those who have gotten dolphin to run on custom installs on a pi 4 have reported that the performance is very bad.
Unless you really have to use a 32-bit OS, you are probably better off using a 64-bit OS.