If you read through some of Alan Kay's other Quora posts there is some more context behind this. It goes a bit deeper than just the tools we build. We don't think like programmers about the act of programming.

Fred Brooks' essay "No Silver Bullet" has a list of past breakthroughs. These are examples of applying programming to programming to ease cognitive load 10x. The essay predicts that we won't see any more of those types of improvements. But we haven't even scratched the surface when it comes to the CAD and SIM tooling for programming. There are many silver bullets left to build.

In a followup comment (starting with "That is an excellent question, and I’m pondering … The examples I gave were examples of the kind of math/engineering I was thinking of.") Alan describes a bit more what he means:

https://www.quora.com/How-does-one-get-as-close-as-possible-...

The entire comment chain is interesting to get some insight into how he's thinking about Engineering.