I don't think that plant was ever only OU
I was a youngish bachur then, and so there's no way I would know all that went on in the world of kashrus at the time -
but:
AFAIK/R, at the time Agri had different runs with different secondary hashgachos along with the OU as well as runs that were OU only.
The fact is that the products found in the camp only had OU on them.
When the rabbanim/poskim involved looked in to it, no one ever claimed that there was another hashgacha involved.
BTW How do
I know all these stories? Well, around the fourth or fifth year I was in the camp some of the maggidei shiur and other ruchnius staff seemed to suspect that there was something rotten in denmark as far as the kashrus in the camp was concerned. They somehow convinced the owner that there was a need for a "mashgiach" to make sure keilim didnt get mixed up. In the meantime I was tasked by them with snooping around everywhere in the kitchen...
So I spent two summers there doing that - around two weeks in I saw enough that I stopped eating almost anything cooked in camp. During the second summer some of the senior staff (non ruchnius) became so concerned that some of them asked their own personal Rabbanim if they were allowed to continue eating there. (AFAIR one or two even left the camp after the summer over the kashrus issue!)