> IMO the issue with the given Postgres example isn't that it is written in C therefore it is slow, it is why it doesn't use all of C's features to make it faster

I think the issue the author is pointing out is that, in the real world, this real-life (fairly obvious) performance improvement was enough of a pain in C, that it was not done for 25 years.

In languages with generics, this improvement would have come for free. With C, yes you can do the improvement, but you have to do it by hand, and you have now to maintain and debug multiple versions of sort.