I think the fact that dragons are powerful psions (able to commune with trolls from a young age as well as obviously having some kind of thermokinetic abilities) is also a part of what makes them not lusii. For the most part lusii tend to be just dumb “””harmless””” animals, and there’s no way the empire would ever officially assign someone an animal with the ability to cut through ships with its eyes and eat megaspiders whole.
there’s also this remark Mindfang makes;
this is a pretty atypical way to talk about lusii, which, as far as we know, share a blood colour within a species. the implication that a dragon would tend to only raise nobles supports both the idea that they aren’t of lusus naturae stock and that the empire would never reasonably let any old person have control over one.
the “in this era” comment is puzzling, but I think it’s pretty easy to explain it as a reference to mythological times; Mindfang can imagine some king in a bygone millenium being raised by a dragon, sure, in the same way we can imagine King Arthur had fought a dragon, but the idea that some slave to the subjugglator council is just riding around on one and cracking down on petty crime? absolutely ridiculous.
most of all the fact that Mindfang thinks she can go out and capture it for herself is what sets it apart from a normal lusus, I think. like having a dragon is less like having a lusus and more akin to having a status symbol.
similar perhaps to trying to obtain an actual live musclebeast.