They have said that it will not be a problem and that mods will be able to be distributed through sites other than Steam (such as civfanatics.com). They have said that Steam will only be required for activation. They have said that the game will be more moddable than Civ IV.
Yes, they could be lying - but if you are going to go that route you would have to conclude that anything they have said could be a lie - including the fact that the game will require Steam at all.
Sorry, but you are missing the point.
Let's stay with Babylon for the moment.
May we assume that Nebuchadnezar will be a different leader from the ones in the standard version?
Most probably, the answer will be yes.
So, what about re-skinning Nebu and issuing a "mod" called "Ancient Babylonia"?
I mean, we are promised to get unprecedented modding abilities.
Do you think, this will be allowed, when "Babylon" will be sold for $10.00?
I for my part heavily doubt that it would be allowed.
Now, let's take it a bit further.
In WL, the Great Wall and the Warlord was invented (and some additional civilizations and so on).
Since DLC has been announced, what would happen if the Civ5 equivalent of the Warlord would be sold as a DLC? Let's call it DLC_2 (DLC_1, for that purpose, has been Babylon).
Would any modder be allowed to make use of that unit?
Most probably, the answer will be yes - since Steam seems to be the right means to ensure that only purchasers of that DLC will be allowed to make use of that modification.
Now, the next DLC may provide you with the Great Wall. Let's call it DLC_3.
The same principles will apply.
And finally, there will be a DLC with the Mongols (AFAIR, they have been invented with WL). That will be DLC_4.
What does this mean for the modders?
You may have the standard installation.
You may have the std installation plus DLC_1
You may have the std installation plus DLC_1 and DLC_2.
You may have the std installation plus DLC_1 and DLC_2 and DLC_3.
You may have the std installation plus DLC_1 and DLC_2 and DLC_3 and DLC_4.
You may have the std installation plus DLC_1 and DLC_2 and DLC_4, yet without DLC_3.
You may have the std installation plus DLC_1 and DLC_3 and DLC_4, yet without DLC_2.
.....
You see the picture?
In Civ4, we were facing add-ons every 18 months, which have set a new base for the modding community.
Chances are good that this may drastically change.
The community of civ players (under the assumption of sold DLC, since these will come with protection) will be split and segmented, thus making the former big modifications (FFH, RoM, and so on come to mind) less likely to be played by a major audience.