With closed eco you have the advantage that the AI cant nuke your economy. Which the just love to do IMO. In the earlygame they just buy all my tools, regardless of how much I build. But with closed eco you have the disadvantage, which ricochet already stated: You need to have the resources in your country / market. For example doing closed eco with sweden in the late game is extremely hard since they have no oil.

But if its just me then how did i go from barely 250 million GDP to almost 1 billion in just 5 years. For reference i didnt build any buildings in that time all i did was close my economy.

It's just you. There are inputs that aren't available in your region unless you conquered it.