“The PIN is more secure because it's really just an unlock code for a hardware private key.” so it is a password for the hardware private key. But rebranded as a pin so microsoft sounds as if it does something to innovate.
It's not really a rebranding because the use of the word PIN here is closer to (and derived from) the use of a PIN in the older, traditional multi-factor sense where for instance a bank card PIN only worked with the associated bank card present. PIN versus Password has almost always implied this sort of multi-factor distinction, and Microsoft if anything is just reusing an old term for what it was meant for.