Indeed it's not an interface to the browser's gc. There does exist a wasm anyref proposal which enables manipulating opaque references in wasm and a gc proposa

Indeed it's not an interface to the browser's gc. There does exist a wasm anyref proposal which enables manipulating opaque references in wasm and a gc proposal for further allocating/using such garbage collected objects. However, basing our gc on those is currently less pragmatic than rolling our own :)