Is there something I'm overlooking?
Two things:-
1) Instructables are by and large crap, written by people who have an inflated idea of their own ability.
2) There are two types of USB device a host and a client. Typically your computer is a host and a MIDI device is a client. With USB you can only have a host communicate with a client, you can not connect two hosts or two clients together. A MIDI synthesizer and an Arduino are both clients so they can not be connected together. The basic plug and sockets try and prevent two clients being connected together.
Now you can get a shield for an Arduino that turns it into a host, called, not surprisingly, a "host shield" so first off you need one of these. Then you need a driver code which you might find on line.
However using a 5 pin MIDI out from an Arduino into a synthesizer is possible.
By the way an Arduino does not use scripts, they are interpreted, an Arduino uses programs, sometimes called a sketch, they are compiled.
EDIT It could be that your synthesizer will only respond to signals on MIDI channel 10. So change that line to:-
int noteON = 144+9; // this is channel 10 as MIDI channel 1 is indicated by a zero