"A folk song is one in which many have taken it up and found variants of it.

I can think of dozens of Folk Songs where this isn't the case."

Please offer examples of this .

In aural tradition in rural areas, songs are taught to people in informal ways.
That's how they get disseminated.

Formal ways may or may not constitute the aural process by which traditions are learned but I see no distinction between the methods of teaching.

Folk traditions can be formalized in their teaching methods by carriers of tradition.

People study music in traditional circles as would a student at a music academy.

Regarding "This Land", not all people from urbanized communities fed on piped in pop music will know this song but it continues to be circulated in other environments and in this instance, I mean this respectfully, it has been around with variants for some time as Woody would have wanted it to be and of course reality is in the eye of the beholder.
The values might be different in terms of what is acceptable or not, but the best teaching is when the student is in the same room with the teacher.