This reminds me of something I made once in "Macromedia" Flash back when I was 16 in like 2001ish or so.
It wasn't really an actual OS, but I managed to get a working desktop with a file browser, fake non functional web browser, and a task bar and start menu.
My flash application didn't accomplish anything, but I felt immensely proud of having been able to create a mock up of a UI, even though it was extremely kludgy and wasn't even able to read or write files in the fake applications.
...No offense to the creator though. I don't mean to compare my high school project to this one. This project is indeed very cool!
I too dream of one day inventing my own kind of spreadsheet and terminal hackery OS, so I salute this person for creating a proof of concept that looks good.