As for Rockstar North/DMA specifically: It was a bunch of nerds making games in Scotland. From having reverse engineered gta3 and vice city and therefore knowing the code of these games quite intimately, i can tell that even at that time (i don't know what exactly was meant by "early") they were still a fairly small bunch of very talented people building the best game together that they could. No huge engines or design patterns, just very straightforward, well or reasonably well written code that does just what's it supposed to. All from scratch, the tooling as well. Of course that's just my interpretation (and maybe i'm projecting a bit) but i imagine it must have been a very fun project for the people involved. Doesn't reek much of management, bureaucracy and questionable practices getting in the way.

I just remembered this video https://youtu.be/7vWSi44ZTSw and it seemed like a chill place with nerds having fun making something. (Actually they were still DMA at this time, not Rockstar).

And as for the achievement, the product turned into a franchise with the biggest entertainment products ever made (GTA 5/6).