Actually not entirely true, DOs can and do most specialties, there are some where there are just so few spots but specializing generally isn't difficult as a DO unless you want to go to Harvard or top tier. And I expect specializing to get a little easier with the merger. Derm is extremely difficult to match period, yes your chances are negligible with a DO but most MDs don't have a great shot either. Aside from premeds and SDN, in real life I've never been given a hard time because I'm a DO.
You have to keep in mind that he's in derm of all things. He's looking at this from the perspective of a guy who wanted to match an ultra-competitive specialty and likely would have been unable to do so had he gone to a DO school. Any advocate of the specialties is going to say don't go DO. And they'll be right, because you're closing hundreds of doors. But in practice it will generally not matter if you are a DO or MD, aside from a few hospitals.