The latter, yes; if there’s no code exercising that path, it’s easy for it to be missed. I do note that although submenus work in XD, I don’t see that submenu support is doc’d anywhere, so submenu support was just luck that the developer thought to add it. Because PS is a different backend, everything has to be wired up again. This will change when we move to a new UI compositing engine in the next few months (this is why some elements will become deprecated in favor of Spectrum UXP). Once that happens, parity is much easier, because we don’t have to have a unique back-end for every single app. That will also unblock animations, transforms, better z-order stacking, etc.
Totally see the value of submenus, of course – just that there’s no test for it, and so it’s easy for one app backend to do “the bare minimum” to support what they know is running on them, and end up with inconsistencies.
Definitely not ruling out submenu support in the future.