That's profit from game sales, which doesn't necessarily mean profit on a scale of the entire store when you account for paying developers, running the servers, getting content to users via CDN, etc. Once Epic is on-par with steam[0], i'd at least expect a 20% epic tax.
If they don't turn a profit it's only because they're paying millions of dollars to game developers to use the epic store and remove their game from steam.
"Sweeney also said that Epic Games makes approximately 5% profit from that 12%, and this could grow to 6-7% as the store grows."
They chose 12% specifically as a balance between profit and undercutting the competition.