no, they[your encyclopaedia definitions are a lot of baloney],believe me,I have been singing folksongs,for fifty years.
folk music can be harmonic as well as melodic,different traditions vary.
the traditions of England scotland wales Ireland use mainly four modes.the dorian,Mixolydian Aeolian Ionian,most of these tunes are suited to harmony,sometimes they are fitted tastefully by leaving out the major or minor third,but with that consideration in mind they are still suited to harmony or chords.
the traditional flamenco music uses different modes,that are suited by flamenco guitar,but it is still harmony.
go out and listen to some harmony groups like the Wilson/OR Copper families.Dick Miles