It says that the decimal is rounded down to the the nearest whole number.
Isn't that
Round off?
If it says "Round down", then you are right. The chance would be 1 in 4294967295, as only 1 decimal number can be rounded to 1 billion.
If it says "Round off", then there are other decimal numbers that can be rounded to 1 billion. In this case, the chance would be 3 in 4294967295 which is equal to 1 in 1431655765.