Well it shouldn't be at a loss. As the person I responded to pointed out, Epic had a lower fee than Steam so the developer can sell on Epic for less than they would on Steam and make the same amount of money.
Doing so wouldn't be at a loss, but it wouldn't make as much profit as possible.
If the developers did choose to sell on Epic for less than it would bolster the Epic store and potentially lead to more people moving to Epic.
If Steam's fee is 30% and Epic's is 15% the developer could sell on Steam for $70 and make $49 and they could sell on Epic for $60 to make $51. That's a 4% increase in profits.
If the Epic store takes off and a large enough user base switches they could maybe increase the Epic price to $62.5 which would result in an additional 4% increase in profits.
Epic's deal is that they're offering a lower rate, but the developers aren't sharing the benefits of that to help Epic grow. If they did the long term profits would likely exceed the short term.