they're going to buy OTC and dump spot market in an effort to control the dollar price of Bitcoin. Just look at what Coinbase has said with respect to their reported $7,000,000,000 USD of OTC traded the first day ETFs did.
Do you really think this method can have a considerable effect on the price? Coinbase is the biggest bitcoin [fiat] exchange but it only has about 4% of the total market's trading volume. It is not longer 2013 where one exchange (Mt Gox) has 85% of the total volume where manipulation like this were easy, you just had to dump in one place and the rest followed whether they wanted it or not.
If you dump in Coinbase, the arbitrage traders would thank you but the market wouldn't really move.