The base was VP10 + tools from Thor (Cisco) and Daala (Xiph.Org/Mozilla), and it evolved from there to AV1 through a process of "experiments", test, IP checks and then its enablement.
Assuming that VP10 shares a significant base design with VP9 it would not be surprising if some part of VP9 sillicons decoders could be leveraged on customer hardware, while awaiting for more dedicated circuits.
But on the software end, libaom (AOM reference implementation) is indeed a fork of libvpx. But this library is not broadly considered a good implementation, even for VP9.
Pehaps the guys behind EVE for VP9 [1] will produce an AV1 implementation based of their codebase.
Assuming that VP10 shares a significant base design with VP9 it would not be surprising if some part of VP9 sillicons decoders could be leveraged on customer hardware, while awaiting for more dedicated circuits.
But on the software end, libaom (AOM reference implementation) is indeed a fork of libvpx. But this library is not broadly considered a good implementation, even for VP9. Pehaps the guys behind EVE for VP9 [1] will produce an AV1 implementation based of their codebase.
[1] https://www.twoorioles.com/eve-for-vp9/