This is how they can hack cars with onboard computers, through the lights, or any other sensor. The same principle allows them to form DNA such that when it is scanned by a computer, malware is written by the computer itself that infects that computer. Physics makes the rules, not code. Every input is also an output, and vice versa.
Every commercial chipset is backdoored on the factory floor. Mechanisms like the Intel Management Engine bypass the boot process, encryption, and every conceivable user means of protecting their systems to lay bare whatever they do with their hardware. Researchers are busily reverse engineering IME and other such mechanisms now.
This is why Open Source Hardware is necessary to any system with the potential to be secure.
That doesn't mean Snowden didn't provide useful information, and sacrifice his career to do it. For me Snowden merely verified what I already knew, from other sources. For many people he was the vector for learning they were subject to unlawful tyranny and the panopticon.
The nature of the enemies of freedom provides us certainty that every possible means they can use to surveil us they will employ. The NSA has never once told the truth about their surveillance, and they didn't start now by telling us they're not watching anymore. They're creepy stalkers by nature, and the paucity of actionable intelligence forthcoming from phone metadata won't change that.