I don’t know enough about the ins and outs but I would have thought they’d have to disclose this, and more to the point, someone would have noticed a holding that large appearing on-chain - can’t see Blackrock having thousands of wallets to hide such a large stash and don’t know how they’d have done custody under the radar.
Discussion
Vanguard doesn't have to disclose anything. Its whole purpose it to guard ownership without disclosure.