He's wrong, but I don't think it's slander. How great the risk is debatable, but it's fair to worry that Mamdani may lose the D's some centrist and (although it can't be said out loud) Jewish votes and, consequently, the election. Which would be disastrous. AIPAC certainly is not going to be dumping money into Mamdani's campaign, and if they instead chose to spend it on the Republican candidate, it could sway the general election.
However, I think Trump proved that extremists can win, so while it's a reasonably concern, evidence suggests it's unfounded. Personally, the possibility that AIPAC could focus efforts on defeating Mamdani as he represents a crack in the political wall of unwavering support for Israel is a far greater concern, because money wins elections.
Although a disagree with the characterization that this constitutes slander, I do think it's the wrong approach. Having won the primary, if Democrats want to win this thing they have to solidify behind the nominated candidate. Being divisive will only help them lose the general.
The republican candidate is Curtis Sliwa; we could do a LOT worse. The guy was defending Mamdani on fox news the other day. What does that say about the dems?