These numbers would only be incorrect if market prices aren't "right." The problem is, markets are always right, unless there is some manipulation involved. If that would be the case, then there would be even more reason to avoid BFX. After all, it wouldn't be more than a scam-platform.
Perfect price requires two things - perfect information and infinite demand & supply.
There's no perfect information here - how can an average lender compute the chances of default? He doesn't know how bitfinex's finances look like. He doesn't know how the future btc prices look like.
There's no statistical model to make. Just a mostly irrational hunch.
The second. Even given perfect information, the price is right only if both the supply and demand side is infinite. In real world mispricings are everywhere, and they only disappear once someone actually takes 'impossible' profit.
The whole thing about probabilities tells us that no price is perfect, because an event either happens or it doesn't. On a perfect market with perfect information the probability of default today would be exactly 0%, until the actual default day, at which point it would be 100%. If you have other odds it means you don't have perfect information.
Especially liquidity providers should be aware that by providing liquidity at BFX they are running a bigger risk than by buying into Bitcoin.
Barring theft, serious incompetence or similar circumstances, the only possible situation in which lenders can lose money is serious, fast price drop where there's not enough demand to close the trades without lenders' loss.
In this situation, bitfinex can and should just give the lenders bitcoin it can't sell. If the price doesn't rise afterwards, they would lose dollars, but their situation would be either equal to holding bitcoins or better (if bitfinex is able to return part in dollars).
Stopping trading in this situation, which is what happened already, is exactly that - bitfinex was waiting for enough liquidity.
Note also that bitfinex can, in a critical situation, sell bitcoins from margin called trades on other exchanges (or just wait for arbitrageurs). So what really matters is the entire bitcoin's potential demand, not just bitfinex's orderbook.
Thus, lending is less risky than holding bitcoins.
The reverse situation for btc swaps is theoretically possible but I don't think it's a remotely realistic scenario.