Chaos magic in DC is definitely of the same tier if not higher then Chaos in 40k (mostly relevant in Marvel Crossovers, because Scarlet Witch gets a huge power boost in the Justice League/Avengers crossovers at the cost of her sanity due to the difference), and that's Fate's primary foe.
The main thing holding Nabu back here is that he, as a Lord of Order, would be doing 1 v 4 versus the Chaos Gods. Nabu is strong, but I don't think he could win alone, even with a powerful magical vessel like Magnus. Maybe if he joined forces with the Star Child.