I am firmly of the opinion that "folk music" has nothing to do with rhythm, chord structure, subject matter, the instruments played, the age or source of the material, or any stylistic details. It is to do with context.

Thin Lizzy singing Whiskey in the Jar to an audience of thousands was rock music.

Me singing Ace of Spades unaccompanied in a pub session is folk music. Ditto when one of my mates sings "Delilah" and everyone in the pub joins in the chorus.

The context is this: that the musicians and singers are participating for their mutual pleasure in the music. The songs and tunes are ones they all know or in styles they all recognise. It is an activity, not a performance.

That's "folk music".

A "folk song" or "folk tune"defined by its source and is still a folk song or tune if performed in a rock style (e.g. Whiskey in the Jar, House of the Rising Sun) but it is not "folk music" if it is performed in a non-participatory context.