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The boot menu option doesn't actually run inside Win8. It's actually a completely separate OS environment, except the files happen to live on the same partition as whatever OS you enabled it from. The reason you can still restore the partition where those files reside is because the Rescue environment loads entirely into RAM, which allows that partition to be overwritten even though you booted from it. But that environment is also NOT limited to being able to restore only your Win8 partition; it can perform restores (and for that matter backups) of any partition on any disk that it can see. In that regard it's just like having regular Rescue Media, and the reason is that you're actually booting from the exact same set of files that Reflect uses to create Rescue Media, so there's no need to enable another boot menu recovery environment in your Win10 installation.
On a side note, make sure you still keep external Rescue Media on a disc or flash drive for emergencies because if your restore ever fails, your boot menu recovery files may not be there for you to boot from again. Same goes if you have a significant disk or file system issue, or are restoring onto an empty disk.