I just tried this in a VM. I installed Zoom’s Debian package, tried sharing my screen, and got the same error about not using a supported Wayland distro, and quit Zoom.
On a hunch, I edited /etc/os-release and changed it to have
ID=debian
NAME=Debian
PRETTY_NAME=Debian
VERSION_ID=9.0
(where previously, it said PureOS and version 8.0). I didn’t change any of the other lines. Restarted Zoom, and tried screensharing, and it worked.
So basically, Zoom doesn’t bother checking whether your system is actually capable of screensharing, it just checks if the distro name is in the list of supported distros. You can change the distro name and have it work, but I have no idea what sort of other problems that could introduce (maybe it’s harmless, maybe it breaks updates, I dunno).
If you really want/need to use Zoom’s screen sharing on PureOS, then you could edit that file every time you want to share, then change it back to the PureOS values when done.
Or you could set up a VM with one of the supported distros, and use Zoom from within that (though then you’d also need to access your presentation/whatever you are sharing from within the VM too, as it only shares the VM display, not the host display).