"The spot market is so thin" I also think is incorrect. GDAX is normally 0.01 USD wide (albeit for small amounts) and often-times under $1 wide for many BTC. I would call the spot market very thick, if anything. You can get off what appears to be very large amounts of BTC with minimal slippage most times of the day.
"didn't seem to take much open interest in the futs to whip the underlying" -- are you saying a futures move would lead the market? I respectfully disagree. All the entities I know price futs from spot and are generally putting on the arb in this order, not the other way around.