Sorry, but that is all anyone seems to argue about here.
Generations are meant to be vague somewhat, that's the whole idea of them. Even Wikipedia are pretty murky in how they define them; in their opening paragraph on article Millennials they say the following "Researchers and popular media use the early 1980s as starting birth years and the mid-1990s to early 2000s as ending birth years", noting 1981-96 and later 1982-2000 as major ranges. Therefore ... seeing as nobody can quite decide where even the EXISTING generations start/end ... the grey areas where generations meet (= the cusps) has got to be pretty vague, even vaguer than normal generations.
Personally, I would say that the closest you are ever gonna get to defining a cusp - some would dispute that a cusp even exists, but whatever. that's a whole other matter - is by limiting it to the people that can claim either generation, based on what source you are using. Therefore, I would say Zillennials as a label could be claimed by anyone born in the mid-1990s through to the early-2000s (as per the standard definition of where Millennials ends), while Xennials could be claimed by anyone born in the late 1970s-through the mid-1980s. And so on.
I see a lot of people born in the mid-1990s and early-2000s who dislike that they're not always classed as cusp. As a 1999-born, I can see either as peers, thus this means that either one can claim it; nobody has a monopoly on it.
Hope that cleans up the cusp mess for good.