The joke was not about software specifically, but about the whole system, everything. But even software feels like evolutionary forces are at work - when you work on huge systems developed and added-to over years, sometimes decades, often by new people (lots of churn, contractors), the "design" is less and less visible and it becomes a mess, the role of deciding whether a new "gene" (feature/big fix) works is taken by the (also messy) huge test suite and since nobody understands the whole system any more people code to pass those tests. I made the comparison looking back at when I once was part of those adding to a huge existing piece of software without understanding (it had become impossible, the only objective was to have the tests pass, or even just most of them).

When I later also took biology classes (and org. chem., biochem., physiology etc etc) I could not help but see parallels - I say parallels, not transferring the models over! - to how we develop huge systems, be it software, hardware or the combination of both. No single human understands even a significant part of them any more. "Deliberate design" is not the sole force working on those systems any more.

I'd add that most software systems compete on the market, which is as close to direct evolutionary process as you can get in modern environment. And that process has a fitness function that's quite misaligned with what a designer wants at any step of the process.