It’s an intentionally misleading tweet.

To simplify, It’s saying, if the genome were a string of some length, for any possible byte b and byte position i, there exists a person whose genome has b at position i. Unlike normal character strings, there are only 20 or so valid characters (proteins) encoded by 3 “bytes” (nucleotides) each. So you’re looking at O(10 billion) values of (i, b) that would still keep the string well-formed in its 3-bytes-per-character encoding.

Most importantly, it’s not saying anything about having some bytes b1 and b2 at the ith and jth positions or any generalization thereof.

Human genetic code has limited number of letters. If all people had the same set of letters all people would be identical.

Numerically there's enough people so that each differs from that hypothetical singular human by at least one letter.

Of course there are a lot of caveats to this.