Well, maybe "ran away" is wrong, but one second, he's there, talkin' to Delenn, and then she turns towards Lennier, who was approachin'. When she turns back, Neroon is gone.
No, it was not yet time to confront them in a combat mode. As he said to Delenn, he was giving her a chance to change her mind before taking more drastic measures. The quarrel was with her and not with Lennier. Lennier is no pushover, but he would be no match for a memebr of the warrior caste so accomplished as Neroon was.
I'll let ya know if my opinion of Neroon improves.
I guarantee that it will, though it won't be until late S4.