But wouldn't the coding of the AI be covered by Gamebryo?
Gamebryo doesn't code the AI, devs do. Garbage in, garbage out.
You have to understand, where you see a game engine, I see compiled C++ code. Same with the AI, I see code. The binary is only as good as the coder. It's like playing a low resolution video on a media player. Because the video is low resolution and looks like crap, it doesn't mean the media player is bad or that it can handle a high resolution video.
I would love to know what this guy means by "AI problems", because as far as the meshes and animations go, the game engine will play them. Since he's saying "AI problems", I am going to guess the issues would be with random NPCs encountering a ladder in game, and not with the main character going up and down a ladder.
As for the engine being "old", well, sure it first came up a decade ago, but again, it is C++ code. Source code can be edited and recompiled. Coding a new game engine from scratch wouldn't make much sense. Most game engines are written in C++, so it is only a matter of adding whatever new resources are available, edit the code, and recompile.