The license of the previous code didn't magically become the GPL altogether, instead it by default became un-distributable. They were required to (a) stop distribution of the existing code since it at best had no clear license, and (b) if they wanted, going forward remedy the license by clearly making it GPL or doing the rewrite of the dependency. Or even reach out to the library author and ask for an LGPL or other alternative - there is sometimes (often?) some flexibility there.

The built-in conflict resolution in the GPL is no-distribution.