You can in theory keep it cornered in its own room (sandbox), but there's no way to be 100% sure it'll stay there. You can be absolutely sure that it will never stop trying to get out.
App Store apps can be trusted to stick to a sandbox because they are vetted to a standard before being allowed there.
There's no reason for Epic-F-Droid-Store have their applications adhere to any kind of standards enforced by Apple. They can do whatever shitty manipulative tricks they want to try to escape any OS level sandboxing Apple tries to build.
Doubly so for 3rd party browsers. A browser has a huge attack surface and can also run code (javascript and WASM at the least). Keeping it secure and sandboxed is not a trivial thing.