Why do you assume he didn't? It's quite possible to have this conversation one on one and then use it as an opportunity to address a wider audience. They are not mutually exclusive.
I conclude that he didn't address the student personally and directly with this message from the fact that he expressly framed the public response as a message intended for that student as well as others, rather than a public recitation for others of a message he had delivered to the complaining student in person.