If this is the Raspberry Pi Desktop for PC I notice that the ISO is named:
2021-01-11-raspios-buster-i386.iso
So I am wondering if they may be using a 64-bit kernel (hence the cat /proc/version output) but on a 32-bit system? (ie using a 32-bit system similar to that used by RasPiOS on ARM). I believe that the AppImage does not run on 32-bit.
There are unofficial versions (such as Flatpak) but I don't think that any of them will overcome the fact that the Raspberry Pi Foundation has built their Debian Desktop OS as 32-bit and Joplin requires 64-bit.
Assuming you want to keep with this OS a check of the RasPi downloads server shows only rpd_x86 so a 64-bit Raspberry Pi Desktop for PC does not appear to be an option either.