"The 'source singers', of course, did not use the term,"
Some did, some didn't, but most of them had their own names or means of identification for the songs we call 'folk' (see relevant threads).
"nor did they necessarily distinguish between 'traditional' and 'commercial' songs in their repertoire,"
Yes, they most certainly did - this has been argued ad nauseum elsewhere and apparently remains one of the great myths that haunt this subject. I woud be interested to learn on what evidence people who make this claim base it on.
Jim Carroll