It's as good a theory as any (I don't particularly have one) but I can't imagine why they'd put in human sized seating areas or vertical joins between passages that you have to stand up to get through if they were for animals.
All descriptions I've ever seen of the choke points / bottlenecks ('schlupfs') indicate they were consistently aligned for vertical transitions. This has always made me wonder whether they were a clue to something relating to a fluid or vapor level within the tunnel(s).
For example ... If water were readily available, flooding lower areas of the tunnels could effectively seal off the higher areas. The problem with this (as a general explanation) lies in the fact many of the erdstall segments get progressively deeper as one moves away from the entrance.* For such cases, flooding would only yield a markedly inconvenient cistern.
*NOTE: It's never been clear to me whether there's any preferred, much less dominant, pattern of elevation change (upward versus downward) in transiting the schlupfs as one moves deeper within the tunnel(s). Some get progressively lower, whereas some have both up-schlupfs and down-schlupfs (in series, moving away from the entrance). I've never seen a depiction indicating a consistent upward trend. Then again, only a few of the erdstalls have ever been rendered in a 3D style.
In a similar vein ... The single entrance and poor ventilation motif wouldn't be so weird if the tunnels were used for smoking meats (whatever ... ). Carry in the goods, arrange them, build and tend a smoldering fire back toward the entrance, and wait ... However, I've not seen consistent reference to (e.g.) evidence of fires or smoke residues like one would expect if this were the case.
The vertically-transitioning bottlenecks could have served as crude but effective portals to block animal movements. Some animals won't willingly try to venture down a vertical hole. It would only take a single modest wood post / chunk inserted into the vertically-oriented hole to effectively lock down any animals on the other side.
The pictures I've seen of the alleged 'seating areas' are consistent with seating having been the builders' intention, but I've not seen any example that couldn't be interpreted as serving some other purpose (e.g., bases for cross-timbers supporting a horizontal shelf or platform).
As I mentioned earlier, I'm still uncertain whether all the erdstall tunnels really represent a single phenomenon in the first place.
