Game devs have many teams all with different jobs, for a big game like this you'd typically have multiple teams dedicated to optimization in different areas (and between them). The specific problem in this case was how the game was communicating with graphics drivers (among others), which for any graphics heavy game is very fundamental to performance optimization. The problems aren't even an after-the-fact optimization sort of thing that teams should have to identify and follow-up on, batching jobs is standard practice when interacting with GPUs whether or not there's a translation layer.

When the devs of a core translation API between two supported graphics drivers that are commonplace in the gaming ecosystem have to write code to specifically fix issues with your application you've done something fundamentally wrong.

A lot of posts like these also seem to imply that the open source community should somehow be less competent than these companies and are surprised that the open source community can fix these issues. But the open source community has a ton of very respectable and extremely smart developers, it shouldn't be any surprise really.