The vast majority of people who play this game will still be waiting for that elite 5% or so to do it for them. I guess I just wonder why they're making it a main selling point and going on about "unprecedented," "unlimited," etc., when this is above the heads of the vast majority of players.
While I understand your basic point, I think you're underestimating the "possibility" factor, which ties directly to the consumer confidence.
That is, there are a lot of people who have ideas for mods they'd like to see. In many of these cases, the issue really boils down to knowing whether what they want is even going to be possible for that top 5%. I'm not talking about a simple gameplay mod like boosting the power of certain units by one or two, or adding an existing ability to another unit. Back in Civ 3, I'd give all archer-type units a range 0 bombardment attack. This did nothing on offense, but if a stack/city containing the archer and a spearman was attacked, the archer would fire a free shot before the spearman took the brunt of the attack. Apparently, it was such a good idea that Firaxis added this in a later expansion. But it was a trivial change to make, and anyone could have done it.
That sort of change should be trivial for anyone to do. But for Civ 5, the mod I really want? If I own a tile that a combat is taking place in, all of my units in that tile should get a +10% combat boost. (This'd have a HUGE impact on the ability to invade another civ; not only would their cities and chokepoints be harder to take, but the defender's counterattacks would be stronger.) While simple in theory, this is essential for my work on a future-era mod, because otherwise the game would end through conquest long before reaching the new content. And yet, I have no idea whether this sort of change is even possible for modders to make.
Or another part of that same mod would be to remove the Spaceship as a victory condition, and have it instead trigger an event (for ten turns after launch, everyone's in a golden age and all wars are paused for a cease fire, but the civ who launched it gets an extra tech, a massive diplo boost, a new Natural Wonder, and a longer golden age). I have no idea whether this sort of event system is feasible for a mod.
Likewise, if you go over to the Ideas board, there's a great thread on adding religions back in to Civ 5 by way of "Holy City States", like the Vatican, that found Civ4-style religions and spread it to the main civs. This'd be a huge boost to gameplay, but simply put, no one knows if it's even possible to do something close to this.
(And that's not even counting the people trying to remake Master of Magic or Alpha Centauri using the Civ engine, which seems to come up every time a new Civ game comes out. But hey, they DID remake Colonization in Civ4, so who knows?)
So when Firaxis brags about unprecedented modding ability, it serves its purpose: it makes me think that it's more likely the sort of mods I want will eventually be possible. If the only things people can mod are basic unit stats, maps, and unit artwork (which is the extent of the "moddability" of most games), then half my ideas won't work and it makes it far less likely that I'd stick with the game in the long term (say, long enough to buy the inevitable expansions).
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