This story makes no sense
Why would he carry the file himself when he could use a cloud service?
I have doubts about him being a spy because I myself did the same 10 years ago. I mean I traveled to and from the United States with several hard drives. t was the easiest way to continue my developer work on the other side of the pond.
He did not declare the second laptop? You mean the one he had porn on?
He has a spare SIM card? Of course he has a separate one to use in China. I would not think about a SIM card as a data storage device. Not nowadays. How much it has? 32k bytes?
And the one way ticket. It may be cheaper to buy a two way ticket from China to the US than the other way. He could have planned that. In fact he could have traveled to China on the return part of his previous China bought ticket (which would be a one way ticket only to the suspicious minds of the feds). In fact I always traveled on tickets bought outside the USA.
I'm glad I stopped working for the American firm shortly after 9/11.