This "disappointment" has already made more WW than BvS, a far more "sure thing" franchise film than a family movie sequel practically a generation on from the original could ever be.
It also cost slightly less to produce, and probably to market too.
To get anywhere near Nemo's levels of attendance - remember, we're comparing with one of the most beloved films of an era where computer animation tentpoles were rare and almost always huge - in the US was already impressive. To come anywhere near even the same raw revenue numbers in Europe with a release window that, by sheer accident, could scarcely have been worse-placed if you tried? Yeah, BV have to take that. Throw in a useful showing in China, and you have yet another megahit for a studio that is having one after another lately.
A delayed sequel like this, especially for a family movie, is never, ever a slam dunk. Heck, BV showed that themselves with Alice 2, and that had half the delay! Dory, though, is going down as one of the biggest animated hits ever.