I'd lean the same way as you (just a hypothesis from me too). A .app on MacOS is just a special kind of directory, so the compression covers the normal file types inside of it.

Localization files compress well, compiled code compresses well, repeating assets (@1x, 2x, 3x) and the pair of binaries in a universal app (x86_64 + arm64) do too, etc.

Ah, and dmg compression is just LZFSE, zlib or bzip2, so pretty standard stuff as far as I understand it.