Example: Someone makes a billion $ bet BTC will be $5,000 Dec 31st. They are trying to effectively short BTC. If BTC on coinbase/exhanges is $10,000 on Dec 31st. This person who made the bet just has to pay the difference between $5,000 (their bet) and BTC physical price of $10,000. So they owe $5K in fiat.
Nope. That's not right.
If they "
make a billion $ bet BTC will be $5,000 Dec 31st" then (depending on exactly what you mean by "make a bet") they probably have a contract to sell 200,000 units at $5000. If the exchange price is $10,000 on Dec 31st, then they need to pay the difference
PER UNIT. They don't just owe $5k in fiat. They owe
$1 BILLION in fiat.