[throwaway992673]

Oh yes, I checked caching, checked that the URLs were official by comparing them to the URLs CentOS canonically suggested, checked that the URLs did include security update repos, definitely dove into commands for "force check updates" type commands and the kernel rpms just weren't there. Old ones, new ones, nothing.

Now I could have made a mistake, and according to the article text CentOS 7 "isn't EOL yet" so if someone has a mirror that does work I'd be interested to see evidence that kernel updates are even available. I'm not checking myself though, I don't trust Oracle enough to risk spending time standing up a CentOS 7 machine.