If ETH is over-hyped and does drop in price, investors would not want to sell back their BNT for ETH because they’d be losing money.
The two currencies aren’t truly pegged — only pegged if BNT cost drops below the cost of ETH because of reduced demand for BNT, relative to supply. Then, smart contract would buy back BNT as soon as it’s available and ensure that it stays at par with ETH. Seems clever. That’s my understanding of it, at least.