Ok so I was playing infected to find these two guys sitting here impossible to kill

Screenshots:

 

They try to jump from the ladder to the fences. That is what I have been told.

 

Yes, the jump from the ladder to the fence is possible, though quit difficult, as a human. The players show could have done that, but standing at the edge of the fence and getting "boosted" up by a zombie (hit or attacked by the zombie and using the little jump that occurs when damage is taken as a boost to be pushed up a 1.5 or 2 block ledge) is another possible way to reach that place.

 

you can still kill them though.