I don't think there's anything telling them they HAVE to be exclusive just because they use Epic's software - they own Unreal Engine. So if you made your game with Unreal Engine, you normally pay a royalty to Epic if you sell that game on Steam or some other service. If you sell that game on Epic's store then they wave the licensing fee and you still get 88% of the profits.
However if that's what the devs are claiming, then it's still complete bullshit of an argument since according to Epic's FAQ they don't require exclusivity to have that waived:
"Beyond the normal ease of shipping on every platform, there are no engine royalties for UE4-based games sold on the Epic Games store. If you’re using the default free license with a 5% royalty, for example, you won’t pay the licensing fees from your sales on the Epic Games store. Royalties from sales on other platforms would remain the same 5%."
So if they're claiming they HAVE to, it's because they absolutely do not want to pay royalty fees to Epic for Unreal Engine for selling the game elsewhere.
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However if that's what the devs are claiming, then it's still complete bullshit of an argument since according to Epic's FAQ they don't require exclusivity to have that waived:
"Beyond the normal ease of shipping on every platform, there are no engine royalties for UE4-based games sold on the Epic Games store. If you’re using the default free license with a 5% royalty, for example, you won’t pay the licensing fees from your sales on the Epic Games store. Royalties from sales on other platforms would remain the same 5%."
So if they're claiming they HAVE to, it's because they absolutely do not want to pay royalty fees to Epic for Unreal Engine for selling the game elsewhere.