Actually they are looking into adding modding support and yes, it seemed like they got surprised. Which is funny because Firaxis should know by know that people like to mod their games even though Civ V modding is still very underwhelming compared to Civ IV. I have personally given up on it in sheer frustration about it. The most glaring deficit being singleplayer-only. With that in mind I would not expect any better for modding support in XCOM if it ever happens.
Actually these things were cut under time constraints. But it could have been part of a DLC, yes. It's however unlikely because it would be unusual to leave code fragments in.2) Things that modders have been able to do, particularly things like unlocking Second Wave, was intended to be unlocked as part of a DLC further down the line. Firaxis is big on the whole DLC idea, and I wouldn't put it past them to go "so we've got this feature done that could extend the replay value of the game significantly, let's not release it until sales start dropping".
Although it is true that mods are user-generated content and may collide with DLC plan I believe that as a developer however you should strive for excellency in such cases rather than going the mediocre way and have ordinary hobby modders surpassing your DLCs in quality and features. I know Firaxis specifically did have a more relaxed stance though on that in Civ V seeing their map packs being ordinary maps created with the official map editor. The only exception were new factions and the latest DLC which in fact is not easily moddable via their own tools at all.
So I do not believe they are truly afraid of being outpaced by modders, it's just limited resources because making a game fully moddable requires some sort of API and documentation, sometimes even an editor. Sometimes the limit is simply given by middleware license restrictions but I don't believe that to be the case here. They simply did not reserve a resources for modding at all - mostly because it's a multi-platform game and they are still a relatively small developer (120 employees) - and thus it's now a postponed thing again.