I agree that for a song or ballad to be considered traditional, it must be transmitted orally and be sung by non-professional musicians (ie. 'the people', das volk, etc.). It may or may not have a known author. To a certain degree, there should be evidence of recomposition or textual changes specific to various times and places where the song or ballad was sung. A lot of traditional material was printed in broadside form, but many more broadsides never made it into the tradition, or they died out. The question of whether a 'resurrected' lyric set to a new or existing tune is traditional or not depends upon whether that revived / revised song enters the oral tradition. I would say the same thing for recorded hillbilly stuff from the early 20th century. Some was tradtional in origin, some was newly composed but entered tradition, and some remained commercial and non-traditional.

Whether or not the author is known is much less important than what happens to the song or ballad after it is created.