I stand corrected, although from a policy-making perspective ScummVM allows users to run software acquired through ways not sanctioned by Apple (or owners of the original IPs), which I believe is the major part of why emulators are not allowed on App Store and thus bears a similarity to them in that particular aspect.
Apple controls the JS sandbox in Safari, whereas they don’t control the ScummVM scripting sandbox. In theory it’d allow the execution of arbitrary scripts that can do anything native code can do, although I don’t know - maybe ScummVM checksums the games you load into it and will only run code it recognizes (and if so, maybe that’s why it’s allowed).