I mean even with the help of his research team he still thoroughly looks into all the material he mentions on his show. He constantly calls out the exaggeration from the left when they call someone a “Nazi” or racist.
So that alone would be enough for him to visit the site. He clearly gave a nod to Vdare (which has been named a “white supremacist” website multiple times). So the fact that he didn’t do the same for DS means he’s actually visited & witnessed the atmosphere.
He probably figured he could use the DS as a similar example as Vdare but obviously he couldn’t. To avoid all complications, he says he knows nothing about this site but continues to use it as a tool for defending free speech.