If, for the sake of argument, you wish to define folk music as song with a guitar accompaniment played in D with no amplification, that's OK. As long as you state your definition before you enter an argument. One can define folk (or any other kind of music) in terms of musical style, historic context, time period or whatever. There's NO single accepted definition for "folk'.
    Some people feel that if a song is traditional, it matters not in what style it's played or sung (Steeleye Span, Mormon Tabernacle Choir, whatever)--it's folk. Others feel that if the music is sung or played in a traditional style, it matters not whether it's recently composed or not (Fiddler's Green, Shoals of Herring). Sill others feel that if they like it, it must be folk. And some imbeciles still blather about singing horses.