Not necessarily the case here.
Steam's revenue split is between 70% to 80% going to the dev based on sales.
Epic's split is 88% to the dev.
You don't have to be a genius at math to see why this is a mitigating factor for Ubisoft in case D2 gets bad sales, or even if they have an overall lower number of purchasers by going Epic exclusive. But the other side of the story here is why they don't put it on both marketplaces--and this is explained if they believe that customers will not make a distinction between the two (if true, easily supported by data which we don't have but Ubisoft does). Basically, why let these customers buy from the less profitable storefront at all? And again, even if there are less customers overall, it could still be made up by the different in revenue share.
So yeah, I don't think moneyhat is the clear reason here.