Yet here we are, discussing the secret OS that runs on its own chip and can't be disabled by the end-user.
The world is a pretty ridiculous place, even with the best efforts of those ridiculous FSF zealots. I don't always agree with them and their methods and communication skills aren't perfect, and it's a shame to see coreboot/libreboot get fragmented etc... but imagine how backwards the world would be without some people taking things 'a bit too far sometimes'.
Another difference is that Coreboot is essentially a rolling release, whereas Libreboot periodically takes a snapshot of upstream and maintains it for a while. Libreboot is therefore something like a long-term service release.
Anecdotally, I also found Libreboot easier to build. There is less configuration involved.
https://www.coreboot.org/Intel_Management_Engine