Do you mind telling us what is your current version of Linux Mint? You might need to upgrade to the latest version of Linux Mint to install the latest Electrum version.
Looking at linux mint forum[1], i'd guess OP use Linux Mint version 19.1 or older. And it looks like Linux Mint before version 20 already EOL[2].
I think this is a compatibility issue according to the error it seems there is no GLIBC_2.28 in your current OS.
The app image is supposed to contain all dependencies. It does not rely on the host OS to meet dependencies.
In theory yes, but in practice there are some assumptions for what the host OS has and libc is one of those.
In general with AppImages, the idea is that the developer is supposed to build them on a very old Linux distribution, and users are then able to run them on newer (but not older) distros.
The AppImages for 4.3.2-4.5.2 are
built using debian 10,
which ships glibc 2.28.
Debian 10 is already "oldoldstable" at this point, so OP should really upgrade their system.
And in general, there's always FUSE[3] and Sandbox[4] problem which affect few Electrum users[5].