Sure, you can introduce a minor DLC that includes plague mechanics. Go for it. But, what happens when you want to introduce another DLC that enhances the plague mechanics or introduces new systems that assume the presence of plague mechanics? Then, you have a few options and none of them are good. You can give everyone the plague DLC for free (lost revenue, potentially upset buyers). Or, you can include the plague mechanics in the new DLC (same problems as before, plus a "useless" DLC hanging around). Or, you can provide fall back mechanics for players that don't have the DLC (potentially not possible, depending on what the mechanics are). Or, you can limit thew new DLC's audience to only those players that bought the plague DLC (lost revenue). All of those options kind of suck. Paradox usually goes with the third option and it's made a mess of their big games. And, yes, they do have to balance for every possible combination of DLCs. How else would it work?
Anyway, I think there's a pretty clear distinction between Civ DLCs and Civ expansions. If you can't see the difference, then I don't know what to tell you. There's no sense in parsing words for technicalities.
The original idea was to provide a full expansion in smaller DLC packs. I'm arguing that that simply isn't going to work. A single plague mechanic is not a full expansion.