snowdonia I suspect not much in terms of grapheneos, but could it mean that an otherwise obsolete device could be repurposed for other uses?
Yeah, I guess. If mainline kernel fully supports the hardware, it would probably make it somewhat easy to port mobile Linux distributions to that device, and for Android forks to keep the kernel updated with security patches in the foreseeable future. The device will still not receive any firmware updates, but the security of the device probably does not hinge on that, as it is common to treat firmware as untrusted, and isolate it with IOMMU and similar. But it would indeed not mean anything for GrapheneOS, as GrapheneOS require all firmware to be actively maintained with security patches.