I could be wrong, but I don't think he's defining "dirty" as "against the rules".
My personal take is this. (and I will put on my flame suit now) I personally believe that hits (checking) should be strategic. I personally hate the idea of going out with the full intent of causing someone to be carried off the ice. I understand that I'm operating in a moral grey area here...but I can't help how I feel about it. To me, the intent to cause harm, the intent to injure someone to the point of taking them out of the game is always "dirty" in my book, regardless of whether or not they are following the "rules".
TLDR - I hate "head hunters" on principle.
EDIT - I want to clarify that I did not always feel this way. When I was in my 20s, I followed the WHL Spokane Chiefs when they went on their 1991 minor league championship run. We had an "enforcer" named Topperoski who was THE BEAST of the entire minor league. Not a great defenseman, but when he was on the ice, he was only there to hurt and intimidate people. I loved him in my 20s. But I hate the whole idea now. Play the freaking game.