After some reading, I see what you're talking about.
So I stand by what I said, in the context of what the OP suggested. Containers are inherently tied to the kernel of the host OS. You can't host a Linux container on windows or macOS, because they dont run the linux kernel. This is why they run in a VM - even if that VM is transparent to the user, it's still there.
Microsoft added APIs to Windows, to allow it to host "containers" in a similar fashion to the Linux kernel, so you can have a "windows container", but again, it's tied to the windows kernel so it has to be hosted on a windows host.
So the only way you'd get "macOS images" to "run on ECS" is if (1) Apple added container hosting capabilities to XNU (the kernel used by macOS) (2) Apple changed their EULA *again* to allow hosting containerised instances and (3) Amazon adapted the software running ECS to also work on macOS, and provided this service.
So it's not just "if Apple would allow it".