That's not really true, a QR code pops up, and you can use your other device to read it.

Even if your other device is a mainframe computer thousands of miles away, so long as you can photograph the QR code, you can decode it and send the data string for the passkey device needs to it.

The issue that many have with passkeys is that someone will tie a passkey to an ecosystem that they don't personally control like Apple's and then they can't get into their services without an apple device that's onboarded into Apple's ecosystem.

The other device has to respond "in nearby radio space" via Bluetooth LTE when using the QR code bootstrap, but that's about the only requirement and it can be just about any device.