It's completely wrong on two levels. The first is that very high rates in the past were not due to lenders' risk estimate but because of insufficient supply (the interesting thing is as bitcoin price rises swap supply shrinks because some lenders buy bitcoin, possibly with margin). This is consistent with the fact that rates are dropping for a long time now.  
The second is because the information to evaluate risk isn't available to majority of lenders. Thus the risk as perceived by 'the market' is meaningless. It's obviously riskier than government bonds, sure, but that only sets a very low floor.  

There's also another thing, some people keep money on exchanges to be able to buy bitcoin fast after some event, bank transfers can take even a week from some countries. For these people the rate is not related to risk at all (which they already accept), but to perceived opportunity cost of having to wait (at worst) 2 days to buy bitcoin.  

It's one thing to be an exchange, but they're also offering anonymous interest-bearing short-term deposits. It would require a lot of very costly red tape (like a banking license...) in most countries, probably turn out to be impossible in many.