Yeah he has guests like that but they're playing some pretty non-metal music a lot of the time. Even sometimes when there is a very metal bit of music it ends up sounding like a sample, not the way it would sound on any metal album. I mean... people call Eminem rapcore. But musically I think it's all straight rap. They might express some uncommon lyrics for usual rap but to me it's all fair game for rap as a style of music. Like the band Body Count I would consider to be metal, because their music is totally metal even though their vocals and lyrics are extremely outside of usual metal styles.
I do believe, however, that those guys have had some metal bands in the past. Anyone ever heard that stuff? I don't doubt that they've done metal before but I think the death rap music has a metal kitsch rather than a real metal element to it. But I could be coming from a biased angle, since I've heard a lot more metal than rap. But it's not like I haven't heard very much rap.