Of course you can default on a loan collateralized with BTC. And if you do, they can keep your BTC as repayment. Just like any other secured loan.

The increase in price of BTC has absolutely nothing to do with the loan. You could've just taken out a traditional loan and held your BTC, and you would've made the same return. The loan didn't give you a 70% return, lol, the bitcoins you own did.
Again, everything you describe sounds just like a regular old loan that has been around forever. This is like a few years ago when every company just added the word "blockchain" to their name or mission statement to sound more valuable. You just took out a loan, but they worked "bitcoin" into it so it sounds like some new groundbreaking product.