That's a tough one.
Three things gave me that opinion:
1) He's spent the past 25 years pissing on his musical legacy with Dio. His work on Diver is stunning and your description of why is perfect. Yet prior to that article he's always treated it — and the metal genre in general — with disdain. He basically said that's not art, that's shit — something I did to make money. As a huge fan, it's tough not feel personally disrespected.
2) He's squandered his talent in a background role in the soulless corporation that is Def Leppard. At times he's alluded to the music as not being satisfying, but the paycheque being good. That makes it pretty tough to respect him as an artist.
3) The shit he talked about Ronnie went over the top. He could never articulate it very well beyond name-calling, but as near as I can piece together, their dispute was as simple as this: he wanted to be an equal partner in the income from Dio (the band), and perhaps in the music as well. Ronnie said "no." Campbell's frustration was understandable, but it's not as though Ronnie stole from him, or changed the rules. If he was so stifled as an artist, why the hell is he in Def Leppard? If it was purely business, why not be more professional? But his response was to throw a 25-year tantrum and slag Ronnie at every opportunity. My 13-year-old daughter shows more emotional maturity.
I want to believe Campbell is finally realizing how he was wasting his time wallowing in bitterness, and that his former "hatred" for the music was actually just a defense mechanism to deal with his guilt and frustration over his departure from the band. I believe in redemption and I hope this is what is happening here.
However, his track record could also suggest this is simply another cash grab from a mercenary dickhead.
If I fail to give him the benefit of the doubt, am I just using common sense? Or am I also guilty of being a douche?