That doesn't seem odd to me. Google's data hoarding is done in software, not hardware. Remove Google's add-on software and you have a more or less blank slate to work with. I don't see why we'd expect any different.

This is the answer. Google play services and related privileged components are the non-open source blob hoarding data, along with whatever backend services you use from Google. These components are part of the stock android image that comes on the device that's replaced entirely by GrapheneOS.

Naturally if you continue to use Google services then the data hoarding continues.