Do you know why he suggested an optical isolated led circuit? It doesn't seem necessary to me. I don't know anything about the script stuff (I'm unclear if the referenced article is supposed to be arduino code or midi side code) but I am familiar with led driving. In this case you probably don't want or need multiplexing, it adds complexity and noise (which is probably why the opto isolator) without much value. Latching shift registers are the best choice for what you want to do. In this case 3 arduino pins control 8 leds, but by adding a second shift register (and some code), 3 pins can control 16 or more leds. It isn't very fast usually but for human display purposes it is plenty fast enough. There is a tutorial for arduino here (probably one of many and the first I found in a search):
https://www.arduino.cc/en/tutorial/ShiftOut
and this is a basic sketch (schematic only) of the circuit with the connections for a second shift register for 8 (or less) more leds. but without leds (as they connect same as the first one).
3pins_8leds.fzz (12.9 KB)
Peter