Such gains by limiting included headers is surprising to me, as it’s the first thing anyone would suggest doing.

Yes indeed. I think this is a testament of the loss of know-how we're seeing in software engineering in general, and how overambitious but underworking developers try to stake claims in technical expertise when they even failed to onboard onto the very basics of a tech stack.

I'm sure it's a matter of time before there's a new post in Figma's blog showing off their latest advanced technique to drive down build times: onboarding ccache. Followed by another blog post on how Figma is researching cutting edge distributed computing techniques to optimize build times by replacing ccache with sccache.