At least from the Indian experience, I’d wager that tons of iPhones from the US find their way into other countries because it’s way cheaper to get it from there than pay local prices that may include higher customs duties, higher (than US) taxes, etc. It’s possible that a lot of iPhones in Cambodia are/were originally sold in the US.
Everyone back then had third-hand used stuff that I suspect was mostly ex-eWaste. Smartphones were not ubiquitous.
I see the graph but don't believe it, especially given how much of their economy is tourism.