Given that we seem to be in an ongoing escalation spiral towards WWIII I'm amazed at how little interest people take. The Ukrainians pulled an attack like this off literally last month, and it was widely publicised how they did it. If the Chinese military wanted to pull something similar off they'd ship components in and assemble them inside the States. And then probably make an attempt to recruit other Chinese drones act as decoys in a large swarm to overwhelm any anti-drone defences.
The more obvious counterargument to the idea that is relevant to the DJI restrictions is that there is likely no special need to use DJI drones, arguing that out would require a better knowledge of exactly what anti-drone defences the US has then I possess. But the feasibility of the style of attack isn't in question.
And it may well be that easy to smuggle a massive amount of bombs into the US, their border sounds pretty porous. Based on the stats theoretically an army could have walked in to the US over the last couple of years and they might not have noticed. Around 11 million people are in the States after bypassing the official checks and at its height the Eastern front in WWII was only around 10 million soldiers at any one time if we add the Germans and Russians both.