By that interpretation, their calendar versioning (19.1.0) is also valid semver, because there's no rule about NOT just incrementing whatever whenever.
true, but doing that makes no sense, because people who made their software depend on major v18 are not switching automatically, because they first need to check if any breaking changes happened and if they need to modify their own software. Semver is "semantic" for a reason - to simplify this daunting task. Increasing major version even when there are no breaking changes is technically still semver, but without the helpful warranty, so it completely misses the point