Hi there @Drakkar
Sorry to read that you found the mastering part of this tutorial lacking information but IMO you're focusing on the less important part of this course. Like the tutor mentioned, the main goal of this "How to Make" tutorial was to show how to make a synthwave genre track and I think he did pretty well in that regard. Crafting the individual sounds & arranging the all mix is very well explained and the presets + midi files as well as the FL studio project files available in the course resources really help to achieve this.
Now back to mastering, it doesn't have to always be something extremely complicated. If the mix is already sounding good and if you took care of your levels and some EQing, it's just a matter of cutting unwanted frequencies, final EQing and boosting the levels. So yes, you can get away with a single stem & a relatively simple mastering chain in the end.
Not sure what video you're referring to when writing that something was done between the end of the track arrangement and the final stem export to .wav and that reverb all over the place before it. He's not even playing the same part between video 10 & 11, so it's difficult to judge this IMHO.
