Amazon security experts took a closer look at the flagged ‘U.S. remote worker’ and determined that their remote laptop was being remotely controlled – causing the extra keystroke input lag.

With access to the final remote desktop, and access to the workers laptop you know the delay from these two so if there is more delay, then you can infer it's coming from somewhere else? I'm sure there are more paths too but access to the North Koreans hardware doesn't seem required

No. I'm talking about measuring the time in-between inputs being received over the remote connection. Purely observation from the receiver side of the connection.

Network overhead + dropped and re-sent packets, introducing unusual lag in between commands/keystrokes.

A key being pressed and key being released are two separate events that get transmitted separately and usually happen pretty close together. That gap getting larger, due to the long-distance connection introducing lag, could be what they were looking at.