1. All traditional stuff is folk. Call it folk(1).
1.1. Folk(1) is folk(1) regardless of how it's performed.

2. Some other stuff gets called folk - call it folk(2).
2.1. Music that's called folk(2) is generally performed in certain ways (e.g. the solo acoustic guitarist, the guitar/banjo/fiddle combo).
2.2. However, music that's performed in those ways doesn't have to be called folk(2) and very often isn't.

More generally:

3. There is absolutely no rhyme or reason to what does and doesn't get included in folk(2), and you'd go mad trying to find any kind of "airtight definition".
3.1. It varies from decade to decade, year to year, week to week, and needless to say from country to country.
3.2. Some people regard this endless, omnivorous openness as a good thing, or even as the distinguishing feature of folk(2).

A couple of other things:

4. The folk audience means the kind of people who like folk(1), folk(2) or both.
4.1. Who these people are, what combination of folk(1) and folk(2) they like, and what they think of as folk(2) will also vary from year to year and country to country.

5. A folk artist is somebody whose work the folk audience likes.
5.1. Folk artists can play anything at all without ceasing to be folk artists - unless they do something the folk audience doesn't like.
5.2. Some audiences will positively welcome folk artists embracing different kinds of material and turning them into folk(2) - see 3.2.

Beyond that I'm not playing.