Maybe they were just referring to the retargeting/relinking in blueprints they showed in the past for custom characters (Adapting characters to use real-time facial capture | Unreal Engine - YouTube)?
There are some people on Youtube who managed to reuse Metahuman face rig for arbitrary characters in Maya (with some issues), but it would be nice to have official files and workflow from Epic. Hopefully they will explain more once it’s released.