DesktopCloneLanguage there are no reasons that GrapheneOS couldn't support the tablet, the project could support tablets now, the issue is and always has been the same as when people ask when/if the project would support other phones in that they MUST meet the projects standards:
Devices are carefully chosen based on their merits rather than the project aiming to have broad device support. Broad device support is counter to the aims of the project, and the project will eventually be engaging in hardware and firmware level improvements rather than only offering suggestions and bug reports upstream for those areas. Much of the work on the project involves changes that are specific to different devices, and officially supported devices are the ones targeted by most of this ongoing work.
Devices need to be meeting the standards of the project in order to be considered as potential targets. In addition to support for installing other operating systems, standard hardware-based security features like the hardware-backed keystores, verified boot, attestation and various hardware-based exploit mitigations need to be available. Devices also need to have decent integration of IOMMUs for isolating components such as the GPU, radios (NFC, Wi-Fi, Bluetooth, Cellular), media decode / encode, image processor, etc., because if the hardware / firmware support is missing or broken, there's not much that the OS can do to provide an alternative. Devices with support for alternative operating systems as an afterthought will not be considered. Devices need to have proper ongoing support for their firmware and software specific to the hardware like drivers in order to provide proper full security updates too. Devices that are end-of-life and no longer receiving these updates will not be supported.
In order to support a device, the appropriate resources also need to be available and dedicated towards it. Releases for each supported device need to be robust and stable, with all standard functionality working properly and testing for each of the releases.
Hardware, firmware and software specific to devices like drivers play a huge role in the overall security of a device. The goal of the project is not to slightly improve some aspects of insecure devices and supporting a broad set of devices would be directly counter to the values of the project. A lot of the low-level work also ends up being fairly tied to the hardware.
If the teased Pixel Tablet meets those requirements then as both Daniel and Gabe have said there is no reason it won't be.