When NYCFC was founded Man City had just started its run (won their first Premier League title 2 years before) weren't the dominant force they are today. Plus there is the whole playing in Yankee Stadium thing. I had always seen it referred to as a joint venture between the Yankees and City. Either way your original comment that soccer fans don't associate NYCFC with Yankees isn't true unless you're talking about people who only follow soccer and don't know anything about baseball.
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.