The developer generates the key.
If that developer then wants to sell through Humble, yes there is a separate Humble split (75% dev, 10% charity, 15% humble), but the developer is free to sell it wherever they want, including their own website or bundled with Itch.io copies, where only the usual variable cut with 10% default applies.
Have to agree that's a great benefit of not signing up for an exclusive deal. But I really don't see how that makes Epic essentially bad or unviable as a competitor, which seems to me the heart of the arguments posted. I do not disagree Epic has any of the problems listed in those posts. I simply disagree that it's bad for business as a whole or bad for video games in general, as Epic's not doing much that isn't already implicit in the system itself.
Steam is nowhere near as innocent as it's been made out to be since this news came to light. And I don't think Epic or any of the devs signing up for their store are harebrained in taking this approach to competing.