You are correct.
Starting in the Windows 10 build 14965, multiple partitions on a usb removable disks (memory sticks, not usb attached disks) are now recognised and can be created / formatted with diskpart.
The primary significance for Rescue media is that you would be able to configure a EFI boot partition (Fat32 required) alongside an additional NTFS partition to store Reflect backup files. This requires two changes.
1) New Reflect logic to allow removable USB media to be configured in this way if the Windows & PE kernel build is recent enough.
2) Making 1703 PE available to the download agent; without 1703, you wouldn't be able to access the 2nd partition once the rescue media is booted.
Once PE 1703 is made available, even without a new release of Reflect, it can be used to with multi-partition usb memory sticks with the proviso that you would need to pre-prepare the usb media with diskpart.
I don't yet have a definite release date for the above two items; it is on the list planned for a release in the not distant future.
I hope this helps.