Re-read what I wrote- I said "make no difference" In other words if the box/conduit/gutter ect is steel, aluminum, PVC, fiberglass, ect it makes no difference, a high resistance splice is trouble (fire) either way (equally).
BX cable doesn't glow red hot because of lightning strikes. It glows red hot due to not having an aluminum armor bonding strip like modern AC cable and not having an equipment like modern MC cable. The interlocking metal spirals oxidize resulting in turn to turn contact becoming electrically restive greatly increasing the impedance of the armor. As such when a ground fault occurs inside a metal box or a fault in occurs an appliance the increased I2R resistance of the metal armor causes it to heat up while carrying ground fault current. Because this resistance also impedes the flow of electrons, the amount of amps seen by the breaker or fuse may be to low to trip the device quickly, or even trip it at all- the armor will glow red hot indefinitely until it ignites a fire.
It is for this reason that NEC 250.118 (10) essentially forbids the armour by itself (alone) of type MC cable being the sole effective ground fault current path unless listed as such.
https://www.snellheatingandair.com/blog/bx-wiring-life-cycle-and-ways-to-replace/
Even THHN, TW, XLPE, ect wire in metal conduit has PVC/Vinyl/Rubber which can become toxic when burned. Metal vs plastic conduit or jakceting makes little difference. Other means like fire sprinklers, fire stops, dampers, exit signs, horizontal and vertical containment, stairs wells, ect, ect are what save people during a fire.
