Well, to me it's only interesting in that it shows they were making up new things as they went and not letting past throwaway lines keep them from telling the stories they wanted to tell. "Heart of Glory" implied that "Kling" was the name of the Klingon homeworld, but then TUC ignored that and called it Kronos (which Mark Okrand later Klingonized as Qo'noS), and that took precedence because it was a bigger story and a clearer reference, and to some extent simply because it came later.
Considering what I said earlier about the common people seeing little difference in who rules them, it's possible that commoners like Kahlest don't see much difference between the old emperors and the current chancellors, and thus use the labels somewhat interchangeably.
Of course, we do know that Gowron likes to rewrite Klingon history to suit his agendas, so we can't necessarily trust his claim that there hasn't been an emperor in 300 years. It could be that the emperorship was temporarily re-established at some more recent point, but Gowron or his predecessors have expunged those emperors from the histories, or at least dismissed the legitimacy of their claims to the throne.