I think you are confusing a nuclear EMP with a “non-nuclear” EMP - or “Intentional Electromagnetic Interference” (IEMI).
IEMI is a localized event caused by a specific device.
You CAN conceal an electronic device that can generate a signficant, even damaging electromagnetic pulse - but it won’t be widespread.
Noise is noise. Blasting anything with a broadband noise source like is produced in a nuke (15hz up to 400MHz from one website)) will find its vulnerabilities. Its susceptibilities.
BTW, this is roughly the frequency range for emissions compliance testing. Susceptibility testing is now done but is still limited (or was limited). Good news is that engineers are doing a better job of designing systems.