They were supposed to be.

by Ragashingo , Official DBO Cryptarch, Thursday, July 04, 2013, 08:05 (3961 days ago) @ TTL Demag0gue

Right. And this is nothing new for Bungie demos. Take a look at the old Halo 2 demo for instance. The Chief was invincible, Jackals were dying to shots straight at their shields, the Ghost the Chief boards doesn't even try to kill him, and all those Brutes that get dropped off barely do anything neither do the two or three dropships overhead. In the actual Halo 2 one dropship overhead or one Ghost or one Brute with a Brute Shot would be a major, demo ending threat.