That link itself calls out that conformant implementations can’t be relied on to call callbacks.

> A conforming JavaScript implementation, even one that does garbage collection, is not required to call cleanup callbacks. When and whether it does so is entirely down to the implementation of the JavaScript engine. When a registered object is reclaimed, any cleanup callbacks for it may be called then, or some time later, or not at all. It's likely that major implementations will call cleanup callbacks at some point during execution, but those calls may be substantially after the related object was reclaimed. Furthermore, if there is an object registered in two registries, there is no guarantee that the two callbacks are called next to each other — one may be called and the other never called, or the other may be called much later. There are also situations where even implementations that normally call cleanup callbacks are unlikely to call them:

Oh! While this one does mention that you don't have visibility, this + weak refs seem to change the game

I remember a couple of years ago (well probably around 2021) reading about GC exposure concerns and seeing some line in some TC39 doc like "users should not have visibility into collection" but if we've shipped weakrefs sounds like we're not thinking about that anymore