One of the points that is made is that since the PDS that's being interacted with here is part of a 'Personal Data Server' rather than the Bluesky product, it ends up able to offer infinite free data storage.

This seems like one of the things that might be part of the references the bluesky team has made at time to introducing a subscription service - providing more space / bandwidth / higher quality video on your PDS seems like the type of hosting that could be offered at a premium tier.