Someone is overthinking and overcomplicating things. This is not Linux :-)

The confusion likely comes from the following:
From the settings of the MCS IDE/View/Compile and Program Options, you have two buttons (Default Path and Find Manually...) to direct IDE to the correct MPLABX assembler.
In the referenced dialog you do set the path to the compiler, ie PBP and not the assembler. There is a checkbox to tell it to use MPASMX assembler instead of the old MPASM assembler but no way to set the path to IT.

When PBP3 installs it automatically launches a tool that should find the path to the assembler. That tool is available in the PBP3 Start Menu folder after installation and can be run manually if needed.

Just run the installer and follow the instructions.