I'm not a crypto guy, so excuse the naive question: Can coinbase loan out money without actually transferring some sort of single-ownership keys for the amount loaned? If so, then indeed, they should be able to engage in fractional reserve "banking".
> and other forms of market manipulation
Modern money markets are based on fractional reserve banking. That's how most money gets created these days. I'm no fan of the prevailing economic system but it isn't some manipulation which moves you away from some pristine state.