The test cases require us to return a decimal number and this sounds like it's asking for a 32-bit number.

I think that your understanding of "decimal numbers" and "32-bit numbers" is not exactly right. Test cases do not require you to require "a decimal number", but just a number. Representation, decimal or any other, is irrelevant here. And 2149583361 can be described as "32-bit number" if it can be stored with 32 bits of information (which it can).