The point is that unlimited supply (fiat) can buy everything and is centralized so its value is stable and declared by the government.
While bitcoin (limited supply) isn't controlled by the gov't which has its own value predicted by its supply and demand through us . I don't know if this answer will satisfy you.
But what is the point of having a money supply that is limited like btc when another unlimited money supply can buy all of that supply
Dont get hung up about the limited and unlimited points. No one person
can buy all of the bitcoin but all bitcoins will be bought (or earned, or traded etc.)
The point is not that bitcoin can be bought with an unlimited currency, the
point is that it is an alternative payment method to FIAT