Being amazed that you can make a human with only 20,000 genes is thus like being amazed that Shakespeare could write all those plays with only 26 letters. It’s totally missing where the actual, meaningful information is and how it is decoded.
The sentence makes exactly the error that it accuses others of making, the '20,000 genes' is not the encoding, that's the codons of which there are 64 some of which encode the same amino acid. The 20,000 really is like a single play of Shakespeare.
The quote is a criticism of a common way of thinking in biology, where many (if not most) practitioners think in terms "one gene does one thing" (or perhaps few things at most).
When one follows that mentality it becomes hard to explain how just 20K genes could make a living organism.
The genome must encode all the components used to make an organism, as well as the instructions on how and when to use these components.
The 'one gene, one thing' or just a few things at most meme died long ago for all biology practitioners that are with the times, the problem is more with IT people attempting to understand biology trying to make computing analogies to make sense of something that evolved rather than that it was designed.
The simplified cartoon image that is associated with DNA, where phenotypes are directly connecting to specific genes - i.e. this mutation makes you smart, or that mutation causes cancer - is not at all how biology works.