Have you confirmed (using dig -x) that the information is really available? Maybe there is some other problem.
Looking at your description and your screenshot, I don't think this affects a lot of users. They typically don't separate (V)LANs at home. This is not to say this is not an important thing, it is just maybe less common than you'd expect.
I think this is a good assumption. How should it work differently? As long as Pi-hole does not know the foreign MAC address, it cannot know which devices belong together.
Yes, I guess that's the issue. If Pi-hole has thousands of users, maybe tens of thousands (we don't know because of the anonymity of the downloads from Github and no Pi-hole telemetry), a lot of unusual network configurations will show up. If the information is not available, how should Pi-hole (automatically) detect which address belong to which device.
As always, just oppnons: I disagree with pseudoclients. It somehow seems to suggest they are not really there. While one might argue that this is the case, they actually physically exist as devices and are no pseudo. In my understanding the term pseudo would cause more confusion. super may not be optimal, either, but right now at least it seems more descriptive for the "it is more than one ordinary client".
Otherwise this is really a cool thing. I just tried it in my network and it seems to work just as expected. I even configured all my devices now and have not seen anything odd.