Nah, that's fake. The point of the Democrats isn't for the center right party to win. It's for the center right party to lose. Being center right is just a means to the end of losing. Because no matter how right wing they are, they won't be able to do as much damage as the neo Nazi party. And that's what the point is.
They're only shunning Mamdani performatively. It's sin signalling. They're doing it to be hated. They might win New York, and that's an acceptable sacrifice if it helps them lose the rest of the country.