If I remember correctly they mostly just entered the base room by the door because MM was opening it again and again to shoot through. Also MM had a second room he could have retreated too. And we all know this base is rather underdeveloped for an end-game horde base, what does it tell us if it gets wrecked?
By the way, it seems they have been constantly balancing the spawn probability of the demolisher, first there were ~8, then none, and now 1-2. Sure, it could just be random variation, but balancing is one of the reasons MM plays the game, so it's quite probable
But it doesn't really matter who is right, the block damage of that zombie makes sure he will immediately have anyones attention whenever he turns up. But that is his purpose in life, he's is a designated "endboss", he isn't supposed to be easy.