MacBook Pro 2012 Bootcamp problem
This is a very common question. To reclaim the lost space, enter the command given below.

sudo diskutil apfs resizeContainer disk0s2 0


Some of the Windows 10 ISO files released by Microsoft can not be transferred by the Boot Camp Assistant to a flash drive as described by the document you referenced in your question. This is either because the flash drive is too small or needs to be ExFAT formatted. 

If you are still failing to install Windows though the use of the Boot Camp Assistant, then consider using the following steps to install Windows.


Use the Boot Camp Assistant to download the Windows support software. In this case, you are prompted where to store the files to be downloaded.
Use the Disk Utility application or the diskutil command to create space for Windows.
ExFat format and 16 GB or larger flash drive and copy the files from the Windows ISO and Windows support software download.
Boot from the flash drive and install Windows. Basically, you are installing Windows the same way as anyone who owns a PC.


An example of one user who recently successfully installed Windows 10 using this method can be found at the question Mojave bootcamp assistant limits partition size.