That's not fact. The Zone is automatons from the portal in the FOS. That is just a device that Jor-el built to accesses the Zone when he discovered it. The fact is the Zone is on the mirror side of all of space time as we know it. When you are in the Zone you are just living in another frequency parallel to ours. This was all shown with Action comics 13 (the Krypto issue). Krypto had been watching over Superman since Krypton blew. If your theory were correct then that would be impossible as the collector had not yet come to Earth bringing with him the projector.
The idea is that one can breach the Zone in many different ways without really needing the portal. Xa-Du did it even while the portal was closed by first coming out using his ecto technology then switching places with Superman via the portal. He was already out put he trapped Superman in there after. Also Superman was able to effect the real world with out the use of the portal by using Xa-Du's ecto tech and his will power.
The main idea is that the portal has very little to do with maintaining the Zone's breachablity as the Zone is everywhere in the universe. So no one kept the door open on Superman's end. I don't see how you can conclude that the FOS portal is the only means to and from the Zone when that has been proven wrong almost each time the Zone is mentioned.