I don't see any practical difference from the pre-T5 state of affairs. Anyone whose family has held a domain for five or ten centuries could be expected to be wealthy enough to buy such plots of land on their own. The only difference is that here the plots are on loan from the Emperor (Which means said archduke would have bought up other plots of land that belong to him and not the Emperor).
Hans
It is not a loan system. That puts too much ownership in the Emperor's hand leading to dissent among nobles. In Dune, we see the noble houses trade worlds based on political favor and power, but by all appearances it is still their world. In the Czar's Russia we saw an extremely powerful Emperor with too much land get killed out of jealousy. The White Army was not powerful enough to withstand the Red Army faction in the rebellion.
In 3I, there is no reason to "loan" property. If the Emperor wants a solid empire it is in his favor to give plots of his enormous land holding to the limited number of nobles. If they do something unthinkable he can always take everything from one of them. Also, he has them on the hook to "lead" not just milk that worlds economic future. If he chooses to grant more than one noble (T5) ownership on a planet he can break it up in numerous fashions (continent, hemisphere, habitable or economic worlds in system). The T5 description is still very high level.
For example, planet X is in Sir Smith's fiefdom, the Emperor gives Sir Donnie on planet X and he is also now a noble of that world. What we don't see in the system that falls on the Ref is that Sir Smith and the population have both requested additional nobles be named for this backwater world, in an effort to introduce economic stimulation Sir Smith cannot justify on his own.
Leadership of Planet X also falls in the Ref's preview. The Emperor gave the fiefdom its not a loan. The second fiefdom could be in a different city or continent without impacting the first noble. Pre-T5 systems are different, of course. A Noble had 1+worlds and that was it. Now Marc, perhaps, thinks everyone wants 3I worlds but he could give out nobility through all of known space (Zho, etc...)