I think this would only apply in a situation where the original author stepped away from an unlicensed project that got forked or taken over, then the people forking it added an OS license (the original author could come back and license the original work under whatever terms they want)

If the original author added the license at some point in the future it would apply to the entire codebase at the time they added it, which they presumably hold the copyright for, so I don't think they could make the argument that some chunk of the codebase which existed at the time of licensing falls under a different license.

I suppose if an unlicensed project gets an early outside contributor, and then the original author adds a license, the outside contributor could claim they didn't consent to the license change (this is why many professional OS projects require contributors to sign a CLA)... but in practice I'm not sure how this would work, since this would mean the outside contributor forked and modified the unlicensed project, so they would have been potentially violating copyright in the first place