Levels are there to create a sense of progression, how is this question relevant to you claiming a level cap somehow has a highly intricate part in making something an expansion no matter how little actual content it has?
Would you consider the addition of a house behind some gates in a town with one quest line with 3 quests about housekeeping an expansion as long as it came with a new level cap? We keep the gates closed to anyone who hasn't purchased the "expansion" of course.