How to guarantee a clean install of macOS Mojave from an external USB drive
Like Allan said in his comment that step specifically says


  Once the USB drive has booted, select "Disk Utility" from the Utilities window, choose your Mac's startup drive from the list, and click "Erase". 


This is one of the most vital steps in the process of a clean install. It COMPLETELY wipes the target drive of all information previously there. If you don't do that all you are doing is reinstalling macOS right over top of what is already there leaving everything that was on the hard drive still there.

If you do that there will be nothing left, no users, 3rd party apps, malware, etc.