There are two types of certificate – development certificates, which allow get-task-allow for debugging purposes and can be installed directly to a device; and distribution certificates, which do not allow get-task-allow and can be distributed through ad-hoc means, Test Flight, or the App Store.
If this app requires get-task-allow, then it needs to be signed with a development certificate to run. If it's signed with a development certificate, then it can't be distributed to other people.
Official description: https://github.com/tbodt/ish#a-note-on-the-jit
It's a full x86 emulator.
> IIRC the dev said he also wasn't allowed to publish it to the App Store
He didn't, it's just never been tested whether it could be on the store or not.
You can compile binaries on iSH and they'll run on the emulator just as all the other code does.