I dunno, Saruman is so knowledgeable in lore that I wouldn’t be surprised that he knew the strongest refuge of his closest neighbors intimately.

It wasn’t really a weakness if you weren’t aware of explosives. If the orcs cut the grate to open the tunnel, it just leads to whack a mole as they emerge 1-2 at a time. If the orcs try to collapse the wall with picks and hammers, it would take an unreasonably long time, and there are multiple counter measures.

There will be a day to point fingers and accuse others of poor judgement…