Or was it a group of Jews, celebrating Passover in the way that Jews do, who are constituents of Corbyn's,
Well, no, it wasn't.
It wasn't his constituency, it was Meg Hilliers.
It was 'a satire' on the celebration.
No one is suggesting they are the wrong type of Jew, except Corbyn's defenders. I think it's highly unlikely that many Jewish people celebrated in the way they did!
He made a decision to go to a seder where traditional celebrations are 'the normal seder shit'. Perhaps he thinks it makes him look edgy. In the context of recent events, it just makes him look like an ignorant fool at best.