> If I give you 64 bits, you can't tell me what number those bits represent

You have to tell me the (non-cheating) programming language that the 64 bit program is written in as well.

> And you're asking, what is the largest finite output you can get from a program in today's programming languages that is 8 bytes or less.

That's what the post ends up saying, after first discussing conventional representations, and then explicitly widening the representations to programs in (non-cheating) languages.