I descend from Elias Sanders through his daughter, Mary Sanders, the wife of John Vickery. Please add me to the list of researchers who do not believe that Elias Sanders was of Indian ancestry. It makes a wonderful story but is not supported by fact. It is rare to find mis-statements in John Baker's book, but (a) he doesn't state it as fact and (b) he very likely relied on stories told by Elias' grandchildren (who were Baker's contemporaries) and who hoped Elias was a Cherokee. The fact that their claim for compensation was denied due to lack of proof of tribal connection refutes rather than supports the idea of Indian ancestry.
I have been unable to find any source for a Plumer tribe, nor was it the case that 18th century Indians were given last names such as Plummer or Plumer. AIthough it's remotely possible he could have been an Indian child who was first adopted by a family named Plummer or Plumer and subsequently adopted by the Sanders family, such a scenario seems highly unlikely and gets us no further toward figuring out who these unidentified Plummer or Sanders parents were. The more likely story is that Elias Sanders was of British descent and born in Maryland to a mother whose maiden name was Plumer. There is a great deal of research and several books on the Plumer family, which was prominent in Maryland during colonial times.