Can we say that steganography ensures confidentiality? I think it does not ensure because it is based on the security through obscurity and then it does not respect Kerckhoffs's principles, but the notion of confidentiality being highly subjective, I would appreciate any answer.
Steganography solves a different problem. Steganography attempts to hide the existence of the data at all, not necessarily protect the confidentiality of the data once it's discovered.
Regarding Kerckhoff's principle, yes, steganography does violate Kerckhoff's principle. Specifically, point 2:
It should not require secrecy, and it should not be a problem if it falls into enemy hands;
Steganography does not use a key, and as such requires that the method of encoding data remain secret.