How was this in doubt? I invent fire, you see it and copy my idea. Technology transfer. The idea is useful enough that it propagates out to the rest of the world over time. This is intuitive.
I don't think there was any doubt, but archaeologists like to show solid physical evidence, of which they had not had any yet, probably mostly because leather decays so quickly. Similar to how it's hard to know exactly how material culture evolved before the stone age because much of the technology and tools have decayed. We have a good idea, and a few examples, but the more we can collect and understand the better we can paint the picture of history.
It's not just people copying each other, we have direct evidence of an Assyrian soldier who moved to China after retirement.
And who then started a DirtTube channel on how to make Assyrian armor. (Sadly the records and backups of DirtTube have been lost).
The article makes it clear they are not sure that this person served in the military or whether they bought/acquired the armor from someone who did after the fact, nonetheless it's a possibility and it's pretty cool.