It is reasonable to see how a black man might be offended by use of the word posse" in this context, but it's not as if most white people are pressed to understand this in their everyday life. Phil Jackson was either willfully malicious in his comments or he was ignorant of how black people feel while working in an industry full of black men. How ignorant do you think he is?
This is a prime example of white privilege. Phil Jackson can survive without needing to care about how a black man might take his words. A black man doesn't have the same margin for error and has to be much more aware of the concerns of white folks.
Everyone also needs to learn the difference between denotation and connotation. Denotation is the literal definition of a word. Connotation refers to the associated meanings of a word that go beyond the dictionary definition of the word. Anyone who points to that literal, dictionary definition as all that is needed to understand the meaning of Phil Jackson's words doesn't understand how language works.
There are many words and phrases out there with seemingly harmless denotation, but have a negative racialized connotation when spoken by a white person to a non-white person. If you are white, you may not understand this because you haven't been on the receiving end of these words while also observing things like tone and body language that make clear the contempt and disrespect behind some of those words. If you are white, you may have used these words and been unaware that you have been expressing subconscious feelings and emotions while using them.