The way we use these terms should probably be democratic, so I'm fine if people disagree with me, but I personally think the term generational in sports terms ought to apply to a once-a-decade talent, because a decade is the outer limit for how long a player is typically in their prime. The most impactful years of two such players will scarcely overlap by those terms. If you use the 20 year measure, then the NHL would only be able to claim what, 4 bordering on 5 generational players? Take the top 4-5 players in league history, and there would probably be extended gaps in league history when none of them were at their peak.
But I have no problem with people who prefer the term be used differently, it's more a matter of taste than anything.