First, there is only one as the second is inside the PC900. The inside one is a current switch on the internal buffer
Second, that's schematic is specifically for T2.0 and are you really using that?
Third, there should always be three things in a short list!
...but I'm fairly sure it's polarity protection in case the MIDI In current has been reversed somehow. And similarly for the T3.x version above on the MIDI page.
https://www.pjrc.com/teensy/td_libs_MIDI.html