They aren’t actually too similar. They both use XNU, but the memory model is completely different. On macOS memory can be paged to/from disk. On iOS it isn’t and applications must free memory when asked or be terminated [0].
iOS applications are sandboxed by the kernel, with no opt out. macOS applications are not sandboxed by default and are opt in.
iOS applications are sandboxed by the kernel, with no opt out. macOS applications are not sandboxed by default and are opt in.
Then there are the API and UI differences.
EDIT: That linked blog post in the parent blog post also shows how different the userspace is: https://worthdoingbadly.com/macappsios/
EDIT: iPadOS 16 enables virtual memory swap [1]
[0] https://developer.apple.com/library/archive/documentation/Pe...
[1] https://www.apple.com/newsroom/2022/06/ipados-16-takes-the-v...