IPMI normally has a dedicated NIC (well, on some crappy boards it doesn't), which tends not to be connected to public networks.

In theory, definitely. :D

Unfortunately though, too many of them still seem to make the connection. eg looking through Shodan quickly just now still shows potentially 1k+ examples. Ugh. :(

But you're right that it's a lot less than is likely present for this AMT problem.