This is the unique development of archaeological hills in Israel, where successive generations mingle ritual objects on their way from the world of the Canaanite mythology to monotheism.”
I don't know how much this can tell us that's new. The Bible tells us that syncretism was a huge problem with both the northern and southern kingdoms.
Or, they were just reusing a convenient chunk of stone.
Convenient piece of stone is the best explanation, apart from the apparently likely possibility that the stone was completely unknown (still buried, and in a supposedly much older strata) when the Second Temple structure was built.