I don't get her impression it's changed much, even playing destiny 2, which in itself is a technology offshoot from reach, likely designed with many of the tools and techniques used in the halo games, I don't feel like I'm fighting wildly different characters of enemies.
I guess for reach , the presence of firefight may mean that there is a separate set of behaviours designed for those maps.
It wouldn't surprise me if the maps themselves are designed with tags and cues for the AI, which may not be there by default. But I suppose this gives the options for modders to try and do it