The far right are so virulently anti-Mamdani bcs they know it'll get the compatible left to even more entrench their support for him even as he's already drifting towards the positions of the center-right party that he's adopted, despite them fighting tooth and nail to keep him out of office.
Everything in politics makes way more sense when you stop believing the kayfabe and recognize they're all on the same team. "It's a big club, and you ain't in it."