You're right that 3DOF needed to exist before 6DOF, which is a much harder problem to solve, doubly so for inside-out. But, PCVR entirely skipped 3DOF as a cons

You're right that 3DOF needed to exist before 6DOF, which is a much harder problem to solve, doubly so for inside-out. But, PCVR entirely skipped 3DOF as a consumer product, and even though Rift shipped without touch controllers it had tracked controllers 6 months after launch. Early 3DOF mobile VR was an attempt at getting more users, without actually thinking about the user experience. Early mobile VR advertisements (for Gear VR or Daydream), had people using 3DOF VR headsets like they were standalone 6DOF headsets with full hand-tracking. People walking around their hands in front of the HMD, etc. Marketers knew how the device should work, it actually wouldn't be insane to say the device that people want is 3-5 years out and not release 3DOF hardware