I’m not good at “the economy”, but if I buy something using part of the value of the thing I’m buying as cash, then didn’t I just buy the thing for less than I said I would? Was this supposed to be a loan?

Not if the deal goes through. Twitter has $X on the balance sheet. Twitter gives $X to Elon’s acquisition company. Now Twitter is worth $X less. The acquisition company now transfers the preciously agreed $44B to Twitter shareholders in exchange for all Twitter shares. The risk is he stiffs them.