I had the same issue. I just now fixed it. In ubuntu 18.04. which I had previously I just installed the matlab application from the Ubuntu Software app. But in ubuntu 20.04 you need to use the Software app
 And there install the package:
 
This will create the matlab.desktop file and from there you can just follow the instructions you linked (adding   -r "edit %f"  after the launch command). Hope this helps.