Everyone has summed it up pretty well.
But the core of the answer is that Slouken was part of the UI team inside Blizzard and communicated with addon authors frequently at the start as already mentioned.
The process was later reciprocal, both the debug tools and secure implementation are in very large part contributions from the author community to Blizzard (Iriel I believe for debug tools and Cladhaire for the secure environment?)
This actually reminded me of a lot of things I'd forgotten and is definitely worth a read.
https://journals.uic.edu/ojs/index.p...load/2971/2529