You are right that the MIDI standard specifies a current loop.
If the transmitter is designed according to specifications, it will send a 5 mA current, not a voltage. No matter which resistor you are using in series with this signal, the current will still be 5 mA through the resistor and the opto-coupler LED.
So why is there a resistor at all? Well, most MIDI equipment doesn't implement an actual current loop, but "fake it" by using a voltage signal and a series resistor. The resistor is likely chosen to minimize the current in case such a common semi-voltage signal is applied, and its value selected to be low enough not to cause problems with weak signals and still give a reasonable current through the LED, while high enough to prevent too much current to flow in case higher voltages are encountered.
The extra diode is also technically unnecessary, but useful for protection against reverse polarity. The 220 ohm resistor helps to dissipate power in this case, so the diode does not burn up.