That’s not true, there are some FM SFN’s in Europe. I think there’s at least 1 in Australia (but I can’t remember where).
There’s a broadcast engineer in Brisbane that I sometimes work with, who knows how to do it properly, & through my processor, I have the ability to run 2 identical feeds out with different timings to suit an FM SFN.
There will be an echo/signal cancelation spot, but you engineer it to be in an area of little significance.
In the cities, there’s reflected signals bouncing around off tall buildings everywhere anyway, giving the same effect as an SFN.