Why would they use a PDS as a source of truth for that? Surely they just have a guy with an excel spreadsheet who posts on all platforms

Poor systems thinking. They'd likely use Buffer or a similar social media management platform. Operating their own PDS means they control the source of truth, just like running their own Wordpress instance. Could they pay someone else to host the PDS? Sure, that works too. Either way, it is about controlling their communications infra.

Ok so why would they use a PDS instead of using Buffer?

What do you do if you want to cancel Buffer in the future? Buffer is the orchestration and scheduler, PDS is persistent storage.