Full size photo - https://photos.app.goo.gl/fgJgkqLgdkMhzvyj9
Thought experiment: What if the Altar Stone is a later, say, Roman addition to the monument? Is there anything that proves it isn't? If it was what would we expect it to be like? How could we tell? Where might they have brought it from?
The answer is probably where would we expect to find debitage from it and it seems it was found deeper than we we would expect from such a late introduction to the site.