You're missing the point, which is that a physical computer in this universe could enumerate no more than 2^500 distinct states. How is this useful? Well, for example, it lets you put hard upper bounds on the size of crpyto keys. For symmetric crypto, assuming your algorithm is a CSPRNG, you will never need a key larger than 512 bits no matter how advanced computer technology gets.

Is that true? Cracking a key is a highly parallellizable operation, where little communication is needed between processors. Would your statement still hold here?

Yes. Just look at the details of the calculation. It makes very (very!) liberal assumptions, and produces a very loose upper bound, i.e. 2^500 is almost certainly a gross overestimate of what is actually achievable even given arbitrarily advanced technology. The point here is that even a very loose upper bound is still quite small.