In the end, "traditional" tends to involve trusting your own ears. Listen to Bob Copper, Leadbelly,Woody Guthrie, Maybelle Carter and May Bradley singing. Then listen to, say, Martin Carthy, Peter Bellamy, Kate Rusby, Jim Moray, Maddy Prior and Seth Lakeman. You will find it easy to distinguish the first group from the second group, unless you've got cloth ears. The first group are what people tend to call traditional singers. The second group aren't, they are people who sing tradityional songs. It doesn't matter if you hear them on youtube, an LP, or your sitting room.
    Some singers, of course, fall in the middle, straddling that awkward fence, and then the arguments can start!