Well, you're not wrong. I'm not sure if it's a bug or developer intent, however. The damage flag for disruptors is set to 7 (aka 1 + 2 + 4) which is bludgeoning, slashing, and piercing damage rather than universal, which would be 8.
For what it's worth, Obsidian changed it to 16, acid, which was a NWN leftover. It could be done for K1. I don't recall if anything else in either game deals acid damage, or if anything grants resistance to acid, though.