I object! That posted on github was just the downloader itself (used primarily by those who wanted to download their enrolled courses), it did not contain supplementary files (certificates) that would help you download even PAID courses.
There is no foul in OP's post. He did maybe create the program (GUI), but the script is not his. And from my P.O.V. what he is really selling is the hardwork he put off in creating that GUI, because it essentially makes it easier or pleasing to use. BUT, I tell you again, the script used here to download PAID udemy courses are free on the web.
You are paying for the graphical user interface, if you buy this program.