I sincerely doubt that the assets on the site in question are large enough to warrant this.

The stylesheet is just under 3KB even with no minification or compression. At that size, the cost is negligible, and inlining will consistently be faster to load even than a warm cache of an external stylesheet. In most conditions, you’ve got to get towards 100KB before it even becomes uncertain. Loading from cache tends to be surprisingly slow, vastly slower than most people expect. (Primary source: non-rigorous personal experimentation and observation. But some has been written about how surprisingly slow caches can be, leading to things like some platforms racing a new network request and reading from disk, which is bonkers.)

Seriously, people should try doing a lot more inlining than they do.