That’s absurd. All it would take is one disgruntled worker to post proof about it. And foreign powers would pay handsomely for a spy or hacking group to provide details. It’s well known that countries like Russia, North Korea, Israel, etc. all have highly talented hacking groups on their payrolls. If they had the slightest feeling that the US implemented such kill switch technology then they’d all be doing everything they could to exploit it. It’s not worth the risk.