> ... explains slope detection using only a diode and a tuned circuit
Slope detection can happen, yes, and unintentional rectification (like
through a corroded connection) can demodulate sufficiently-large signals
(and, BTW, mix (intermodulate), or remodulate, and reradiate).
Broadcast FM signals are not always just FM'd, though; they often carry
a significant AM component, too. AM can be generated anywhere in the
broadcast RF chain, from exciter to antenna. A mistuned broadcast
transmit antenna, for example, will generate AM on an FM signal radiated
from it by the transmit-equivalent of slope detection; the antenna
behaves differently for different frequencies so, as the exciter shifts
the carrier frequency as it should, the antenna works irregularly well,
making AM. A similar effect can cause a broadcast directional radiation
pattern to shift with modulation, also generating AM in receivers on the
slopes of the moving pattern. And, a mistuned transmitter generates AM.
So, the AM you hear on an FM signal might be transmitted that way - and
today, many broadcasters (perhaps most) no longer have an engineer on
staff, so the transmitter might not have been tuned for some time.