Sadly, I haven't really been able to play since I've been having very serious computer issues, that pretty much prevent me from playing on Showdown, so this post is purely speculation that could be totally missing out on important info!.
The thing I take from this is that Defog could be a problem for a lot of Calm Mind Arceus. Ho-Oh just got a lot harder to KO, and with Stealth Rock being harder to keep on the field, a lot of CM Arceus just got easier to check for teams. I don't think they'll be unusable, but in the grand scheme of things, I'd think their prominence will be reduced. So if I were to use most CM Arceus this gen, I'd probably focus on making sure Stealth Rock is actually up often. That could be difficult, but I test pretty much everything, so this is on my list!
(Also, CM Rock, Electric, and Water Arceus just got a whooole lot more interesting to me this gen)