"Efficiency" and "arbitrage" are two separate things. "Efficiency" has to do with information. "Arbitrage" is based upon what people do with that information.

There is also a difference between riskless arbitrage, and risk arbitrage. Most arbitrage is risk arbitrage. How the different players value the risks, and transaction costs, determines if they are willing to do trades to close the spread.

So, even in a 100% efficient market, there are opportunities for risk arbitrage. Otherwise the big guns on Wall St. wouldn't spend millions of dollars to make all the pennies (on what you would consider is a very efficient market) that add up to millions of dollars.

The rise of multi-pools has led to a more balanced market, not necessarily a more "efficient" one. The rise of multi-pools is a simple effect of economics - supply and demand. Not of arbitragers. Someone is willing to pay more (demand), and the multi-pools have arisen to satisfy that demand (supply).

Arbitrage is someone was buying on one exchange and selling on another, and making the prices on the two exchanges converge.