It was pretty much the same as SC2, they just never told you in the UI. e.g. Dragoons do half damage to mutalisks. Yamato cannons don’t kill zealots.
Yeah, I was aware of the different armor types in SC1, but I thought they turned it up to 11 in SC2. Maybe they brought it down to the first 2 tiers of units? It didn't feel like armor types really came into play as early in the game. The impression I got was that you could get away with massing a single type of unit much more easily in SC1.