The point is that the signatures are compared against a database of certified builds - and that exists on Google servers.
If you want AOSP attestation, you need to build your own database to compare against.
It exists on Google's servers for lock in reasons alone.
Which makes sense for them - after all, that makes their competitors break and their ROM doens't.
https://arstechnica.com/gadgets/2018/07/googles-iron-grip-on...
Still amazes me how everyone isn't cynical-by-default about anything Google (or big tech in general) open-sources yet...
(because you still need the hardware made, and it's not like the EU commission is even prepared to fix BSPs for that hardware)
The EU has endlessly sold critical infrastructure to US, India and China while actively sabotaging efforts to rebuild it and now want it back - for free. This is criticized as having a low chance of success, as well as being a pretty unreasonable demand.