I was the team lead to rewrite a large APL codebase with a very small team as part of a much larger group that refused to change and I'll agree with this. Once we started documenting and building tests for the code that replaced APL it became clear that the complicated bits were easy to replace and the hard part was just man hours converting all the conditional logic.

And APL requires supreme discipline to prevent that from happening.

> one-letter variable names

Just like in math!

Except in math, the notation is surrounded by natural-language prose which carries the brunt of the semantic load.