Yes it is an expansion but when people refer to the game they called the base game vanilla destiny and everything else by the name of the expansion. It is frame that way b/c expansion do change the game. Now some may not and only add new content but even that is different from the original. Bungie has even put a lot of importance on the years of destiny signaling TTK as the start of year II and thus a new destiny. This may also be the reason they added etheric light in the HoW giving people one last outing with their old gear and weapons before they are put to bed.