I’m basing this on two things. Firstly, my friends/family know what it is (some of whom are not fans of the sport). I live in an area where MLS teams are known. MLS is mentioned on the local news and in our local and state papers. My team does well consistently in the league which may be a reason why most people in the region know what NYCFC is. Secondly, soccer is one of the most followed sports for Generations X, Y, and Z. There are many polls to support this. Many also may look down on MLS but also acknowledge its existence. If a state does not have an MLS team, I could see then why some don’t know about MLS Cup.
How does any of that help you decide that "most people my age know what MLS Cup is even if they don’t follow soccer"? None of it has anything to do with someone being Gen Y (unless you live in an area that only includes Gen Y people).
I mean, it may cause you to believe that most people in your region know about it (though, I'd suggest that there is a lot of confirmation bias there), but has nothing to with Gen Y.