Would that suggest that a Jeannie Robertson, Jean Ritchie, or Sarah Makem is/was not good enough to perform this music?

Yes it would.

So why refer to them in such a limiting way? A singer of songs is a singer of songs.

A writer of songs is a writer of songs.

A singer who also writes their own songs - is a singer/songwriter.

They may well be the ONLY source of this material but you would not call them a source singer - when perhaps such a description would be more accurate. But it would again imply some limitation to their singing abilities (but perhaps well-deserved in some cases).