That email doesn't say you'd get all the BTC you spent back regardless of how high the price of BTC goes.
Even if HF did say "no matter what" how could you reasonably believe a tiny start-up could possible make good on such an unlimited liability?
Everyone knew bitcoins could skyrocket or crash. It's what bitcoins have always done. So they should have saved some bitcoins around, then there would be no additional liability. Since they never planned for BTC refunds, their offer was a empty promise meant to deceive customers into buying.