Hi! I really hope this doesn't sound arrogant or anything, but I just noticed that the square colour harmony you mentioned beneath the colour wheel should have red-orange instead of red-violet? That would leave a gap of equal size between the four colours, creating the invisible square you mentioned.
I'm aware that you know this perfectly well already, but I figured I wanted to point it out because it kind of bugged me the first time I saw it (I was pretty new to theory regarding artistic aspects). If someone else who are new to colour theory reads this, a possible misunderstanding can be avoided.
That said, this tutorial is extremely helpful and has meant a lot to my own progress in terms of understanding the relations between colours and how I can take advantage of them. So, in case I have not had the decency to thank you before: Thank you very much for sharing this extremely helpful knowledge with us!