He's setting the ball rolling. Firmly plant in the minds of the media (and from there, the people who consume that media) that Momentum is a hotbed of antisemetic villainy that exists to demonise and pressure 'right thinking people'. Pun intended.
Then just wait for the media to push that narrative, so we get calls more widely to shut it down.
He's just showing himself up for what he is. He's terrified of the grassroots. He's terrified of party democracy. He's terrified of the membership being anything other than leaflet-deliverers and door-knockers. He's terrified his game is up.