Well, the Mamdani thing is weird because Jeffries represents part of NYC and has just generally been evasive and somewhat hostile about Mamdani. But yes. There is a barrier between them, although I imagine national Dems have a stake in it that they may want to oversee.
Yeah Jeffries is the one who has a real case for offering an endorsement, but his constituency is a subset of Mamdani's, and they might not be as high on him. I don't know the local politics, but I do know that historically, endorsements can come late to keep it fresh in voters minds at the polls.
And the very excellent reasoning behind that is evident because I can't really remember any of the endorsements he's already gotten. Bill Clinton sticks out but that's mostly name recognition. I actually looked into the congresspeople of MN and WI, only one has ever endorsed a mayoral candidate.