[italicizing added]
sacred means something religious or divine. While Alexander the Great is very famous, does or did anyone who came after consider Alexander to be divine? For example, while people very much admire Abraham Lincoln, nobody would associate Lincoln with divinity.
Another comment says that English may not be the first language of the author, so perhaps 'sacred' wasn't meant precisely. And it could be used, even by an English speaker, imprecisely (hopefully not in published research) or in an exaggerated fashion (also probably doesn't belong in published research).
Still, I find it interesting how a little overenthusiasm and subtle shift in terminology can change perceptions of someone.
EDIT: Better stated: Here is a modern historian calling the sarapis sacred. Why? Sacred to whom?