I ran some tests and according to the sandbox the PayPal system does create a unique Payer ID for each credit card used in the "guest checkout" system.
I went through Express Checkout the first time using Guest Checkout with a particular MasterCard. Upon checking GetExpressCheckoutDetails I do see that I get a Payer ID back, in this case it gave me DKG3GNSK44HV2.
I then did the same thing with a separate Visa card, and it gave me back a Payer ID of QXQGEYLFMBNY8.
Then I did the same thing again with the original MasterCard I used and I got the same Payer ID of DKG3GNSK44HV2 that I did the first time.
Finally, I tried the Visa again and got the same Payer ID of QXQGEYLFMBNY8 that I did the first time with it.
Based on those tests I would say that PayPal is indeed generating unique Payer ID's for any credit card used in their system.
It makes sense because they have limitations about how much money you can spend with any given credit card through Guest Checkout before they actually do force you to create an account, and they also won't let you use a credit card in Guest Checkout if that card is added to an existing PayPal account already.