How I understand it, they wouldn't have to release it if they do it correctly. ODbL allows some mixing with proprietary sources, as listed here: https://osmfoundation.org/wiki/Licence/Community_Guidelines/Horizontal_Map_Layers_-_Guideline
The principle that a map maker can make and publish a map made from several distinct horizontal layers without being obliged to share data from the non-OpenStreetMap layers has been established for several years and is much clearer with the switch to the ODbL. Here we formalise and state what we are happy with and give examples that we hope will help potential OpenStreetMap users with incompatible sources.