Why does "Hodl Hodl" not seem to... support Bitcoin?
It's not a scam. It doesn't allow lending Bitcoin, you are correct. When it says it supports Bitcoin, it means it allows you to use Bitcoin as collateral for a loan.
For example, if you want to borrow USDT, you'd be allowed to post an appropriate amount of BTC as your collateral, receive USDT, then when you pay back your USDT loan, you receive the BTC you posted as collateral back. If you default on the loan, then the lender receives the BTC you put up as collateral.