Not *entirely* correct.
It’s an RF thing, but the holes in the front glass which let you see your food AND the seals around the door are sized to prevent RF specific to the ~2.45 GHz microwaves of the oven from leaking. Other higher cellular frequencies, so-called C-band frequencies at 3.7 - 3.98 GHz, and likely Wi-Fi 5 or 6e *can* get through.
Also, note well, a microwave will NOT protect gizmos in the event of a nuclear EMP. Frequencies are way higher and will get around the door seals and go through the glass zippy-zippy. Plus if you are proximal to the epicenter, the resulting kilo-volts per square meter are so large it’ll induce harmful voltages right through your retail-market Faraday bag. As an example, real Faraday CASES are a minimum 1/4” steel with exotic seals for the lids/doors, and still dielectric nesting (box within a box) is recommended. This of course is for non-DoD spec hardened devices.
The old terrestrial MW networks of the telcos were C-band 6/4 Ghz. They used to use passive plano-reflectors for terrain problems. Thus, I can’t see how 5G 6G, at even higher mmW frequencies, will penetrate a sealed, heavy-duty enclosure, let alone a Faraday bag. Its been a long while since my tech days, so I may be out of touch.