The main point of supplying those two links was in answer the those who had not seen the term used and used in this manner.

Never heard anyone use the term 'source singer', and I assume it means someone who sings sourcey songs

The example from the other link was as follows.

Her grandmother, for example, was the source singer for the well-known song Bunch Of Thyme which is now widely sung.

It also shows that the use of the term in this manner could so easily be avoided by just saying the person was the source of the song.