For the arrow coming from github to the cdn, I would bet that's a deployment where the existing docs site gets deployed as a lambda. From there, CDN calls that aren't cached go to the lambda which are then served up.
If that’s the case, why not just use GitHub Actions? It’s more integrated and requires no setup.
Sure, that's one of a hundred ways to deploy it.
But they don't say if they're using that approach or any other
They are probably more interested in cost rather than setup.
I would assume the lambda is to invalidate / update the cache on the pages that are being changed as new commits are merged in.