Aka when in doubt, increment leftmost, also we won't tell you wether we did a complete conceptual overhaul or just found a corner case that is provably incompatible. And I really thought it would be good.

Not exactly semver either, Angular 4 is backward compatible with 2.x.x. Following semver would mean it should continue to be in 2 series

It makes breaking changes that might break some users. That is following semver. Semver doesn't follow backward compatibility in the major number, it follows potential breaking changes that will impact downstream consumers.