Likely this is driven by ego.

Yann wants to cement his position as a leader in AI and while he clearly does not appreciate LLMS at all, he realizes that he needs to make waves in this area.

Mark needs a generative product and has invested tremendously in the infrastructure for AI in general (for recommendation). He needs researchers to use that infrastructure to create a generative product(s).

Yann sees this going on, realizes that he has a very powerful (research+recruiting) position and tells mark that he will only sign on if Meta gives away a good deal of research and Mark concedes, with the condition that he wants his generative product by end of 2023 or start of 2024.