1. Armored was not a tag on BW.
2. Hydras did not do "double damage" to any unit, they did their normal damage to large units and 75% to medium and 50% to small. Also "normal" damage was 10, which means that they only did 7.5 damage to medium and 5 to small units.
You're absolutely right there, I keep thinking in terms of their minimum damage because of the way damage tags work in SC2, haha. I'll reduce my statement to the fact that all large units from SC1 that are now in SC2 all have the armored tag, and that doing explosive damage is analogous to having a +50% damage vs. armored. It's a pretty weak argument for "swarming =/= no need for +armored damage" now that I think about it more clearly.
In either case, there is no reason to think we need any +armored bonus. Zerg winrate is fine the way it is.