If the phone has a soldered-on WiFi module, that’s when we could end up with the situation where there’s FM functionality on the chip and it just doesn’t get put to use.
But if the phone has a M.2 WiFi card, in a laptop-style M.2 slot, then that card is almost certainly not going to offer FM radio. As far as I know, there is no standard for wiring up FM radio through an M.2 slot.
What I think @TungstenFilament was saying is that you could reverse engineer the M.2 card and inside it there might be one of those chips that does FM radio, then you could mod it to get the FM signals out of it through some wires. Purism could then provide pads to solder the other end of those wires to. I am not sure how serious this suggestion was.