I think he was thinking that turn 5 was the last turn, so didn't bother to do anything with it after he took out the lobba.
And really, my goblin heroes would have probably killed it pretty quickly if it got into combat.
The bestigors fluffed their rolls on my goblin heroes in the first turn that they were in the flank of my night goblins only doing the 1 wound to my night goblin big boss, so that left them in trouble in round 2 when they faced about 37 night goblins plus the 3 heroes.
A 5+ armor save really isn't that great against that many attacks. The only other time I faced bestigors was a horde of 40+ where I didn't have any big bosses in my horde and I still killed 9 of them.
I think he ambushed in the other games, but wanted to place all those little ambushing units on the table for the game against me to guard against the manglers.