Not very likely I would posit. Too much effort to crack the encryption without some super stealthy backdoor method. Not impossible but unlikely as the backdoor to the crypto would have to be pretty beefy or be some sort of "shortcut" in the crypto that somehow has flown under the radar for so long as whole panels of independent experts have been perusing over since the RSA debacle.

More likely and easier to implement would be a hardware killswitch. Turning something "off" is much simpler and cheaper. Hypothetically speaking while I place this mass of tinfoil around my head; One well placed relay in the baseboards or some triggered logic patch that dumps source power to critical components and you bring down whole systems in one fell swoop. Pretty stealthy that way too because it's simple. Simple gets lost in complexity as the complexity acts as noise in the design documents. Government based systems are very much paranoid about this sort of thing so I doubt you would see this in anything like the US nuclear arsenal where they were floating these balloons.

Though with that sort of testicle tight grip on the supply lines in the consumer markets; they could cripple lots of other supporting things that aren't as stringent with their hardware/software sources. Communications, traffic controls, water, power, hell even sewer, so on and so forth. Look at how pissed people get when their internet goes out. Imagine how they react when EVERYTHING goes out and the toilet is backing up! Lots of chaos and turmoil for us to handle all at once!! Emergency response would be paper thin and stressed to the max! Cats marrying dogs outside of wedlock! We're doomed!!!

Ahem... Taking the tinfoil off... The likelihood of such a thing is still pretty darn slim. Lots of hobbyists have been peeling back baseboards and ICs without any permission from the manufacturer and voiding their warranties for a few decades now and nobody has spotted a whiff of stuff like this. We're post "clipper chip" discovery where folks will expose anything for internet clout.

I'm not confident enough to say it's impossible. Just very unlikely they have this sort of capability.

That was my first guess as well. Most likely SIGINT collections looking for "vulnerabilities in the signals". i.e. "stuff we can jam/disable if/when we attack"