Diet Cokes and Door Dash? :'(
This video was actually part of a five hour podcast, so here's the full interview:
John Carmack: Doom, Quake, VR, AGI, Programming, Video Games, and Rockets | Lex Fridman Podcast #309
https://www.youtube.com/watch?v=I845O57ZSy4
I remember this, He talks about using the NeXT for debugging.
The first mention is 1H11M50s and I'm sure there is another mention further in.