Actually, I think that this is a perfect example of illustrating the phrase.
It's the public face/private face dichotomy. And I don't interpret the expression to be speaking in absolutes.
Let's take this example, a man can be a good husband on the surface - supportive, fully engaged with the family, 50/50 in household chores, remembers and celebrates birthdays because that is who one side of him is and he likes to be seen that way by everyone who meets him. A good loyal, family man.
But if he has also cheated, then he is also a liar, he is also disrespectful, he is also self serving and he is also disloyal but these were sides of his character he wished to keep hidden from his wife, his family, his friends and his colleagues.
They are no less 'who he is' than the public face he shows.every day.
The expression is a reminder to not ignore the aspects of someone's character you'd rather weren't there but to consider the whole person.
A person's positive qualities don't negate their good ones. They are equally important and should be paid attention to because, if you don't, its likely to come back and bite you on the arse.