Put windows on a *partitioned* external hard drive?
The link you have provided outlines a method which requires a third party tool to install Windows 10 on an external HDD or SSD. The use of third party tools is unnecessary since Apple and Microsoft together provide all the necessary software required to install Windows 10 on an external HDD or SSD of any intel based Mac that can boot Windows 10 from an external HDD or SSD.
You can either add partitions for additional volumes on the external drive while installing Windows or you can add the partitions after you have installed Windows.
Most users install Windows 10 to EFI boot from the external drive. Many answers have been posted here at Ask Different regarding the installation of Windows 10 to EFI boot from external drives. An example of how to install Windows 10 on a external drive can be found here. You can add additional partitions to the external drive by either of the methods given below.

By modifying the steps given in the link to add additional partitions through the use of the diskpart command.
By using the Windows Disk Management tool to add the additional partitions after installing Windows 10


Note: If you need an ExFAT volume, you may need to first created a FAT formatted partition using Windows, then use macOS to reformat to ExFAT.