Which by definition means it wasn't the only reason, which proves kozikow's comment to be untrue.
It's a commonly cited gripe. I suspect it often comes from people entirely unfamiliar with EU processes, but who don't like EU membership or the idea of EU membership for other reasons and since it's a commonly cited gripe (and also doesn't mention immigrants, and has a pro-democratic connotation) they use it themselves.
It would be strange that no one in this thread has ever heard a Brexiteer cite bureaucracy and corruption, but I fear it's an urge to insist that the only reason that anybody voted for Brexit was racism to delegitimatize leaver votes or even the possibility of debate between two good people on the subject. I fear that attitude will not convert many previous leavers to new remainers in any future people's vote.
I suggest not bothering with arguments against racism, which racists will have heard before and dismissed, looking at left-wing arguments against EU membership from the 70s, and responding to those.