You are right. Any AM radio station can be tracked.

That is all RDF ever did and contrary to what some others have said
here it is very accurate. I use RDF for instrument approaches in
airplanes regularly. The only reason it is not used much anymore is
because it fails to give other information that other equipment can
give these days, like VOR's that can give a bearing to/from as opposed
to just to and DME that is found on most VOR sites. None of that is
really mariner equipment but since aviation is what drove a lot of the
more modern equipment, it is important because it also killed the old
gears popularity.

I mentioned in another post that I tracked lightning with an RDF. The
pulse of electricity that comes from lightning makes the needle of the
RDF jump and point toward the source of the electricity (the lightning)
It is a really great tool should you get caught out in an area of
storms and need to know where the heaviest activity is.

I was out between Swan island, Honduras and Jamaica, right out in the
middle of nowhere, the first week of June one year and sailed right
through the first tropical storm of the year. The RDF I had lead me
away from the strong cells and really did save my life. As for why I
was there, I got stuck going through the Panama Canal and really didn't
have any other choice unless i wanted to spend the summer in Panama and
Colon Panama isn't fun for a day!