You don't need to buy or sell from just other Coinbase users. It can be anyone. Coinbase doesn't have a huge market share yet, so it's even less likely.

I think a fair way to read it is that $1M worth of transactions occurred of which Coinbase made some money.

Not necessarily. I while ago I ventured myself in the stock market and in less than a month I traded over US$ 150k in call options. How much money did I have? about US$ 2k, but I made over 100 buy-sell transactions that summed up to that amount. Unfortunately for me the strategy I was using wasn't profitable enough to outcome the transaction fees :(