Citi is still the exclusive "issuier". Barclays isn't technically "issuing" any new customers with cards, they're just rebranding the existing DM cards with AA ones. Although I'm curious about a couple things.
a) when is this actually taking place?
b) Will the Barclays benefits for DM will now apply to AA (once they are rebranded)? For example the 10K PQM after spending $25K, or the $99 companion certs, or the day pass for the club.