This is exactly the part I don't like in Bitcoin: why is everybody so curious about that happens inside this or that person's pocket?!
I may be wrong, but I think this is about people wondering if they really own what they bought, or if they are among those who don't understand what "not your keys, not your coins" means. Seen from our perspective, they don't really own anything in that case - just a promise that something is somewhere.
A company like this, especially since it's not into Bitcoin, cannot afford to handle lightly such huge amounts of funds.
I expect them use 3rd party that help them handle everything and possibly that 3rd party either provides full custody, either keeps has at least one of the multi-sig keys involved. And I expect that 3rd party does all the actual handling (transactions) and possibly employing yet another company to provide insurance.
And, in order to avoid too many curious eyes, probably the addresses will remain private and the companies hired for independent review will sign strong NDAs.
Of course, that's what I'd do. Are Grayscale wallets known? (I expect they're not, but again, maybe my assumptions are wrong).
