Epic has been very consistent about wanting the % cut lowered industry wide. It's silly to try and depict the Epic Games Store as malicious: Like it or not, developers keep more of their revenue on that store due to the 12% cut, and if other storefront operators don't like that they can lower their cut. When you choose a storefront as a developer you're choosing who to pay off in exchange for access to their platform and customers, it's that simple.

"out of the goodness of their heart" is also a silly way to characterize this positively or negatively: They want a chance to compete with Apple by offering lower costs in exchange for not offering the same feature set as Apple, just like how the EGS on Windows does not offer the same feature set as Steam. They believe developers will pick them due to the lower cut, but that's something the market would decide. Right now the market can't decide, because Apple has decided to prohibit competition.

Incidentally Epic made a public promise that if Apple lowered the cut, Apple Pay users would get the same Fortnite discount that people paying directly get.

How is their game store a walled garden? It can be installed alongside others on the same hardware. Even their exclusivity agreements are time limited.