I mean, mandatory major bumps every 6-12 months means that they would make a major bump even if there's no breaking change. Not sure if that has happened, but that would be NOT following semver AFAIK since major releases are supposed to be only for breaking changes, not for arbitrary time progress. But yeah, if anything I'd say they are very strictly following it.

There are always breaking changes waiting to go in (https://github.com/nodejs/node/labels/semver-major). Often they’re edge cases or native extension things though. Although the choice on when is arbitrary timing, the major bump does mean there are breaking changes.

The nature of JS is that there are very few breaking changes allowed from an end-user perspective even between major versions.