It's not really a "joint" venture. It's a City Football Group venture, with minority ownership by the Yankees to ensure that the team has a place to play, in order to appease MLS.
As for the last comment.... You mean, the people buying tickets to soccer games?
Your line of thinking is irrelevant. NYCFC aren't "the Yankees soccer team" and putting a stadium next to Citi Field doesn't make NYCFC "the Mets soccer team" either.
NYCFC fans aren't going to bat an eyelash at going to a soccer game in a soccer stadium based on baseball team fandom, it's just going to be geographical ease based on a fan-by-fan basis.
The Detroit Lions played 4 miles from the Pistons in Auburn Hills; and then they moved to a new Ford Field next to the Tigers' Comerica Park. The Lions weren't "the Pistons' NFL team" before and isn't "the Tigers' NFL team" after. They're Detroit's team.
Moving from the Bronx to Queens, they'll draw from the same fans as before: Soccer fans who like MLS and aren't New York/New Jersey Red Bulls fans already.