Just played through it again. Now there are a lot of assets, three sections of a city and pretty detailed, although low texture and polygon quality.
So....they licensed the Renderware engine and apparently modified it pretty heavily. That was about in 1999 and the game was released in 2002. As far as I can see they had a team of about 15-20 people overall, they used to be DMA design and were bought over by Rockstar. It seems a mammoth task though creating all the assets/gameplay/levels/coding/scripting in such a small time. What was their secret?