I have no actual knowledge, and it makes sense to assume the PIN is required to implement the EM side channel attack, as without a valid PIN the old, vulnerable Infineon library most likely does not complete all the steps.
Since the RP challenge is not authenticated in any way, nothing seems to prevent an attacker from just preparing a "user verification not required" challenge and getting the Yubikey to sign it.
- Do as many UP-only challenges as required on a stolen Yubikey to extract the private key, not involving the RP (or maybe a single incomplete one, to discover the credentialID)
- Use the recovered private key in an UV challenge against the RP