Semver is a bandaid on a gaping chest wound, that can best be summed up as "major version number changes may break everything, or they may not, but minor version numbers might not break everything, but they may."
It's more of a philosophy than a law, and it can't really be relied on that much; as often the developers themselves can't accurately predict what is a major breaking change and what isn't.
The Linux kernel itself has some baggage from the 2.4/2.6 era that Linus is explicitly walking away from.
