I think that there’s a difference between “nobody fulfills their moral obligations according to utilitarianism, or even tries very hard to” and “nobody believes that utilitarianism is the correct moral theory”. People are motivated by lots of things other than what they believe the correct moral theory is.
But if you have to give yourself and your children the same weights as strangers than almost no one is a utilitarian.
I think that there’s a difference between “nobody fulfills their moral obligations according to utilitarianism, or even tries very hard to” and “nobody believes that utilitarianism is the correct moral theory”. People are motivated by lots of things other than what they believe the correct moral theory is.