Now, this was not a slip in the background. "Feudalism" might have been codified and named by later historians (who simplified actual medieval society quite a bit) -- no one living in it would have called it that, and more recent historians even say that it never really existed in anything like the pure form they presented -- but she could easily have thought of the term, since she was much later than the historical appearance of feudalism on Earth, and later than the historians.
What it did alert me to was monitoring the society and trying to work out whether she was just being sloppy and using feudal when she meant "aristocratic" or possibly "hierarchical." There was one point where a woman ordered another to do something that she had good reason to want to avoid, on the grounds that she was her liege lady. Which does smack of a feudal relationship.
On the other hand, somewhat later, a knight declares he is not loyal to the person who happened to have sat in the throne, but to the throne. That's not a feudal sentiment. Whether the overlord or the vassal dies, the first thing that must happen is that the bond must be renewed with a new oath of fealty between the overlord and the vassal. Loyalty was something passionate and personal, not abstract.