Bowing to other than Allah isn't a sin unless you believe they're divine or you want to worship them.
Ruko' has two meanings, bending over in a position of "bowing," or kneeling down on your knees without prostrating.
Sujod isn't considered as ruko' since it's a separate action in of itself. It's prostrating and touching the forehead onto the ground.
On the authority of Abu Sa’eed al-Khudree (may Allah be pleased with
him) who said: I heard the Messenger of Allah (peace and blessings of
Allah be upon him) say, “Whosoever of you sees an evil, let him change
it with his hand; and if he is not able to do so, then [let him change
it] with his tongue; and if he is not able to do so, then with his