Is the companion iPhone app mandatory for Apple Watch apps?
Yes, a corresponding iOS app is required. The watchOS component is considered an extension by Apple and not a standalone product.

See Preparing Your 
App Submission for Apple Watch:


  App Review
  
  All iPhone apps with a WatchKit extension submitted to the App Store are reviewed based on a set of technical, content, and design criteria. Make sure to use our guidelines and resources before submitting your app for review.