Boot Camp External Drive only able to connect to my computer
The general rule is that the 2015 and newer Macs can no longer boot operating systems using the BIOS method. This is the method used in the installation instructions you refer to. Also, generally 2011 and older Macs can not boot Windows from external drives. Therefore, the installation method, you refer to, generally only works for the 2012 through 2014 model year Mac computers. 

But even if both computers qualify to BIOS boot Windows from external drives, you have the problem of the drivers. One of the steps, in the instruction you refer to, is to install the Boot Camp Support Software. This software installs the correct drives for your model Mac. If you then try to boot Windows using a different model Mac, the installed drivers will be wrong for the this Mac.