The reason why Lucci doesn't recognize Luffy as an emperor is so he can still chase him; that's the whole context of that sentence.
It's a childish loophole, pretty much. CP can't attack an emperor without authorization, but since Lucci admittedly wants to kill Luffy first and foremost, he doesn't recognize him as an emperor and therefore, from his point of view, doesn't need to be authorized in order to attack him again.
Whoever thinks this is a power level issue is pretty much missing the whole point. Both Luffy and Lucci are perfectly aware of who is the strongest out of the two.