Apps that want to JIT need an “entitlement” from Apple (in other words they have a signature from Apple that the operating system validates before letting the APIs work). So the OS supports it, Apple grants that entitlement to Safari so it can use it, but no third party app can get this entitlement.
But surely once "sideloading" (I hate that term, we used to just call it "installing"...) is enabled, apps not installed through the App Store will be able to utilise this entitlement?
