This is simply not true. React 15 is not compatible with React 16 and it can even take substantial effort to upgrade if you used componentWillReceiveProps (though which in hindsight could be avoided in the first place)
I did that migration on a very large react project (hundreds of thousands of lines). As I recall, after dealing with warnings for the later 15.x releases, the upgrade was seamless. They're also good about creating codemods to make the transition easier -- something somewhat rare in the world of software libraries.