The video made it back to China?

You mean as a YouTube? Or, were these covert spies that were flying drones?

How does one know they were flying DJI products? Wouldn't a USA built drone also take photos over a nuclear facility?

Back when.....we used to be able to fly airplanes over nuke facilities. Take all the pictures  you wanted with a 35 mm camera. We used to land at many military bases if they allowed civilian ops. Obviously we flew over airports with airplanes, right down to the asphalt. Take all the pictures you want. Same with "crop breeding fields". 500 AGL or lower, depending on surroundings. Again, you could take all the photos you wanted. Back in the day, Cessna 172s were modified with a cutout in the belly and college kids were hired to snap away with a hi-res 35 mm camera. 172 carried 2-3 college kids because they'd get sick after a while looking thru viewfinder. Pictures were contracted by the USDA and turned into the aerial photos your local ASCS showed  you for set-aside verification. They took photos of every running mile across the midwest. Point is, anyone could've sent photos to whoever taken by much better cameras than most small drones carry.

I'm not sure what the corn stealing situation has to do with DJI drone import concerns.