Robert Graves was certainly capable of stating stating romanticized interpretations as though they were hard facts, but James Frazer was a serious anthropologist. He could indeed be mistaken about some things, since some of his sources (such as letters and diaries of missionaries) were nonprofessional, but it seems extremely unlikely that he would deliberately distort the truth just for effect. If he had an ulterior motive, such as proving that the Christ events may have been fabricated from earlier stories, then there is enough hard factual material to do so without risking his scientific reputation by distorting his data.I think it's largely symbolic, Carla, like the burning of the Carnival King in certain parts of the world. There have been hints of very old ritual sacrifice of a "king" in ancient civilizations but these notions are largely disputed by many modern scholars.
Like others pointed out the concept is a romanticized interpretation introduced by Frazer. Any "solar" hero/god/king "sacrificed" for the common good or for reasons of apotheosis (like Herakles), regeneration etc. can be construed as a "sacred king".