If they’re unhappy with their own API, this could provide an abstraction layer for the app. They rewrite their client to use this API and it frees up two teams to work (almost) independently. The UI team can work on their roadmap while the backend team fixes the internal API or retires the logic and implement it into this new layer.

I’m not saying they’ll do this or that it makes sense. But I’ve definitely done it in much smaller projects.