I agree that OTC could be the way to go but if you are really buying that much, wouldn't buying from the market make it better? I mean think about it, the price is 10k and you want to buy 700 million dollars worth of bitcoin, shouldn't that mean you would increase the market a ton?
I would say OTC is the "safe" method and they probably did that obviously, but market buying would have made it a lot more volatile and increase the price as well.
By "buying from the market" I assume you mean on exchanges? Why would big buyers want to do that?
It would be great for us BTC holders if they market bought $700 million on Coinbase, but it would also make it much more expensive
for them to accumulate their coins!
Big buyers want the price to go up
after they accumulate, not during.