That may very well be the case. The only M2TW mods I've played are Stainless Steel and Third Age. To go a little more in depth, EB1 actually uses both "area of recruitment" and faction-specific recruitment buildings, in conjunction. So what units a faction can recruit depend both on what provinces they control and what infrastructure they have built there. Generally speaking, a faction's most elite troops are only available in it's Homeland regions with top-tier barracks. In their more far-flung holdings, they can only recruit relatively lower quality regional troops. It's different for every faction, and quite complicated, which is why there's a program called Recruitment Viewer which can be downloaded from the official EB website.

Evidently there's going to be some changes to the system for EB2, hopefully making things a little less confusing.