I think "major" was used in the semver sense, which is not how the TS team chooses version numbers.
Sure, we don't bump according to semver, but it's hard to say that 5.0 _isn't_ a major release given how we used it as a way to get a bunch of breaking changes and cleanups in. It's a very opportune time as people will be forced to manually upgrade and acknowledge that something is changing.