Yes, utter nonsense, since if Zadar was again settled by Croats from the surrounding areas, that means that Croats inhabited the surrounding areas.
That is a straw man of a special kind of straw.
Thomas is famous for making stuff up and for his own disdain towards the inland nobility, apart from that, his account is not even a description from the same period.
Split had plenty of Croatian dukes who ruled the city without issue, in fact, during this very mods start date, Split is being ruled by the Croatian noble family formerly known as Snacic and known in the future as Nelipic.
So you are putting a magraviate/march/duchy in the same league of importance as a kingdom?