Hi @BakedPotatoLord and @Peter_Furness,
I think this is the same (or very similar) typing issue discussed in #46522. Linking it here for completeness since @BakedPotatoLord opened that thread and Travis already posted a helpful explanation there.
For now, the practical workaround is to bypass typing with any, for example:
const Texture = (Cesium as any).Texture;
If possible, prefer higher-level APIs like CustomShader or Primitive, since those are part of the public API and have TypeScript typings.