No reason for you to be confused. But if you used your AppleID for the signup, then got an email telling you that you didn’t, that might be confusing.
Nice you already got the card though!
Got similar issues stating to fix my AppleID On my AppleTV, when there was nothing wrong with my AppleID!.
I’m on 13 beta on my AppleTV, but not my iPhone.
I had no issues with AppleCard sign-up.
Could very well be a beta issue if you are on the beta on your iPhone!