Object references are pointers. WasmGC only supports pointers which point to the start of an object. However, some languages have features which require pointers that point inside of an object while still keeping that object alive.

Limiting WASM to what is capable in JavaScript is quite a silly thing to do. But at the same time there are vastly different GC requirements between runtimes so it's a challenging issue. Interior pointers is only one issue!