cobrammer
I have no relationship to GrapheneOS and do not speak for them. The can correct me if I am wrong.
In the past, Pixel phones were used as the reference device for Android AOSP and Pixel OS.
Starting abruptly this year, with no warning, Google decided to make their virtual Android device, named Cuttlefish, the reference device instead of Pixel phones.
Therefore, when the Pixel 10 series was released this summer, Google released Pixel OS 16 QPR1 in binary form only as Pixel OS, and they did not release the source code for the new firmware for the new Pixel 10 devices to AOSP.
When Google did finally release the QPR1 update source code as part of AOSP just this month! they did not include the device firmware source code for the Pixel 10 series (since the reference device for android development is now the virtual Cuttlefish emulator instead of a physical Pixel phone)
However, the GrapheneOS team was still able to extract the firmware blobs for Pixel 10 devices and very rapidly produce a working OS that supports them. Because they are good at what they do.