Bootcamp install on external, partitioned HDD reboots to Boot Recovery Assistant
When Boot Camp installs Windows 10 on a Mac, any Apple supplied drivers required to boot Windows 10 are automatically included in the driver store during the installation process.  There is no outward indication that this takes place. After Windows 10 finishing installing, software including additional drivers are installed. This process is very evident due to output shown on the display.

When watching the video, I did not see any step where the Apple supplied drivers, required to boot Windows 10, were installed to the drivers store. This means the instructions supplied in this video are incomplete.

So you may ask: Why did the example shown the video work? My best guess would be that Windows 10 was installed on an older Mac. Microsoft makes an  effort to include in the ISO all the drivers needed to boot an installation of Windows 10 for any Windows 10 compatible computer. For an older Mac, there would be no needed to copy the Apple supplied drivers to drive store, because the required files would already be in the store. 

Since your Mac is a 2018 model, the drivers needed to boot may not have been included yet in any Windows 10 ISO file. If this is the case, Windows 10 will not be able to boot.

My best advice would be to not try to use third party software in an effort to trick Windows 10 into booting from an external drive. All the software needed to install to an external drive is already included in the Window 10 ISO file.

If you want to proceed based on the video, then eliminate the use of the VMWare Fusion software. In other words, use Boot Camp to install Windows 10 to your internal drive, then use WinToUSB to transfer to the external drive. You then can remove Windows 10 from the internal drive.