Look at this logically from the timelines and the people who knew Robert, and there are some doubts IMO.
RJ died round 38 or so. If my info is correct, King of the Delta Blues Singers was released in 62.
There were still many people who had known RJ and who were still alive in 62. Johnny Shines rode with RJ for 1 to 2 years in the mid 30s and he lived till 1992. Howlin Wolf played with RJ and lived till 1976. Son House lived till 88.
All these blues greats have heard RJ live, and would have had a chance to hear the recordings before their death. These are the ones who gave us most of the stories of Robert. Odds are that they would have brought that point up when the first album came out in 62. Since this is being talked about now, it could just be a marketing ploy to sell more records.
All my opinion, of course.