>>c-hend+Ox
TanStack Start has its own implementation of Server Functions:
https://tanstack.com/start/latest/docs/framework/solid/guide.... It doesn't use React Server Functions, in part because it intends to be agnostic of the rendering framework (it currently supports React and Solid).
To be fair, they also haven't released (even experimental) RSC support yet, so maybe they lucked out on timing here.