How would that help though? If you have a compromised USB interface, and you're entering your pin on that machine, you could just capture the keyboard input anyway.

That's a good point!, it has to be for another reason.

Interested to hear what it's for

Nope, you were right. It's for USB MITM. I guess the assumption is that the keyboard is wired in a different way (a laptop?) or in wireless scenarios (NFC Yubikey).

Quality discussion, both of you, thanks!