While Epic is definitely a shady company, an open platform is still free to choose what operating systems it supports. Epic doesn't support Linux, Flatpak doesn't support Windows, and neither are technical problems that can't be fixed.
Obviously Epic will never be as open as Flatpak, but just because they don't want to deal with Linux doesn't mean they should be barred from any other platform they do want to support.
The Android situation is a bit different they can absolutely support that. They choose not to and went after Google's fees instead, which is a bit harder to defend (but still shouldn't need to be defended, Google should still allow app developers to opt out of their payment processing, perhaps in exchange for footing the bill for storage and network traffic).
With EU regulations coming into effect next year, I think we're going to see big tech companies squirm even more, right up to the point where they'll lose a huge lawsuit. After that we'll either see Apple leave the EU out of spite or concessions will be made, paving the way for installing iPhone applications of your choice.