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.
So the common usage when said ruko' is usually understood to be bowing and sujod would be touching our foreheads on the ground.
Bowing doesn't necessarily have to be a physical act. For example, In 44:23 we see:
Have you seen he who has taken as his god his [own] desire, and Allah has sent him astray due to knowledge and has set a seal upon his hearing and his heart and put over his vision a veil? So who will guide him after Allah? Then will you not be reminded?
In this verse, Allah has stated that your own desires can be your god although we know that it is not possible to [physically] bow to it.
However, in 18:50 we see that angels, did physically bow to Adam under the command of Allah:
And [mention] when We said to the angels, "Prostrate to Adam," and they prostrated, except for Iblees. He was of the jinn and departed from the command of his Lord. Then will you take him and his descendants as allies other than Me while they are enemies to you? Wretched it is for the wrongdoers as an exchange.
This is not considered a sin because the intention of angels wasn't to bow to Adam himself but to Allah as he created Adam. This is clarified in a later verse:
[So mention] when your Lord said to the angels, "Indeed, I am going to create a human being from clay.So when I have proportioned him and breathed into him of My [created] soul, then fall down to him in prostration."