I wonder if some of the reputation of the folks who fought on the Line helped with this. I mean, it was a pretty iconic battle -- Earth's last defense against the invading Minbari, something that most people expected to be a heroic failure even when it was being planned*, saved only by aliens being inscrutable.

* But still worth doing. Says a lot about humans that something like this becomes so iconic.

Knowing that Sinclair was on the Line probably primed Malcom's expectations, especially if he knew veterans from that battle had been easily recruited before.