The rewards from the freeroll will adjust automatically to the movement of the Bitcoin price when it goes up or down, but if the bitcoin price reaches $ 500k then the lowest value that will be obtained from the freeroll is still 1 Satoshi because that is the lowest value in Bitcoin units, or they can also change fractions Bitcoins to µBTC.
That's not really a problem. The smallest part of bitcoin is (currently) 1 satoshi, but freebitco.in can easily introduce sub-satoshi units just by increasing decimal places. It'd be just a minor adjustment. It's not like transactions are done on-chain or anything.