I’ve seen your metal truck travel from your city to the arcology, so you have indeed contributed to its construction. We’ve got more than half the metal required.
As long as you’ve signed up at the arcology, you don’t have to set your trade depot to export metal unless, you know, you actually want to export it. In fact, I had originally set my trade depot to import metal. So an import truck would come in, deposit metal at the trade depot, then another truck would pick it up and take it to the arcology.
If your trade depot is set to export, you’d have the export trucks and arcology trucks competing over the metal, which I’d argue slows the rate of arcology delivery. C’mon, you know you want to see it rise over the landscape. You can make a lot more money from alloy anyway.