Most philosophers are compatibilists, since that’s a good strong philosophical position. Harris is just saying libertarian free will doesn’t exist, but that’s irrelevant since that’s not what people really mean by the term.
The point, please focus on this point, is that it’s not a done deal that all accept free will. There exists a set of people who are smart and believe the opposite. I’m arguing against “everyone should think what I think”.
Edit: although to be honest if you convinced me otherwise I’d be 100% happy! “Most philosophers” (or my own “lots of smart people”) isn’t an argument I am moved by but I’ll go read up. My personal view is that the disappearance of time at the speed of light is incredibly powerful. I don’t feel we’ve explored the implications of that nearly enough. It either means the future impacts the past or super determinism is correct. Is there an alternative?