It would be "2^32", not "2^32+1". 2^32 isn't representable in a 32-bit integer, the same way 256 isn't representable in an 8-bit integer--the maximum is 2^8-1, or 255.
But the argument that they need to represent 2^32 and therefore that's why they used 64-bit integers is dubious. No 32-bit WebAssembly program could ever operate on and pass an object as large as 2^32 bytes because the code and data structures for the smallest possible program would already take up more than 1 byte in that 32-bit address space.
But the argument that they need to represent 2^32 and therefore that's why they used 64-bit integers is dubious. No 32-bit WebAssembly program could ever operate on and pass an object as large as 2^32 bytes because the code and data structures for the smallest possible program would already take up more than 1 byte in that 32-bit address space.