I believe there must be lots of ways for institutions and companies to hide it: special purpose vehicles, subsidiaries, offshore accounts and companies, hiring agents, etc
I don't particularly disagree with this point
As it may or may not be true. If anything, let's assume for a moment that it is and institutions as well as companies hide their investments in cryptocurrencies. But then we won't know that, right? Therefore, their well-concealed investments don't mean a thing to us as we would still be dreaming of the times when institutional money would come to market, even though it has likely already been there for quite some time (as per our assumption)
Each institution has its own financial audit, so they will not be able to hide the wealth they get from the audit, unless the director of the institution intends to corrupt, they have to work together to hide some of their finances, and save it in bitcoin, that's possible..