Well, sort of true and sort of not true, Dick--the fact that time signatures were not used doesn't mean that the music didn't have definite metrical cadences, they were clearly notated in the repeating patterns in the notation--time signatures and measures just made it easier to see them--

The lyrics to folksongs have rhyme schemes and metrical patterns and these are the real underlying "Time signatures"--but, as you say, traditional singers did what they liked with them--there being no reason not to, save if it was for dancing(and a lot of places had traditions of dancing to unaccompanied singing)--