They don’t know the definition of “shirk”, but the fact is different they don’t know the lofty position of Ahlulbayt (a.s) near Allah (swt).
Shirk means if anyone believes any Authority other than Allah (swt) parallel to Him otherwise we believe that Allah is the Supreme Authority and all power belongs to him and He could give power and authority to whom He wishes is not a shirk. And Alhlul bait (a.s) have many more power than anyone can imagine. Their power and authority is beyond our thought.