Hi,
It does seem to be about mail in general.
In some ways AIM is somewhat separated from AOL in that you can register from an AIM Screen Name separate from an AOL one.
On the other hand the AIM Registration page used to allow you to list any other email as the AIM Screen Name (you could in fact register a Gmail ID as the AIM screen Name just to confuse things).
Some United States ISPs also have their email addresses allowed and AIM Screen Names.
The Notification does seem to list Apple Issued IDs and not just any iCloud linked Apple ID.
The process was that you could obviously have an @mac.com ID based on the paid for .Mac service and link this to an @me.com from MobileMe at the change over of those two services. Then when iCloud came out you could initially change it to an iCloud one and later Apple issued @iCloud.com IDs in addition for early iCloud adopters and now as standard.
However I have an external ID to Apple and have made that my iCloud ID.
It has given me an early @me.com and an @iCloud.com IDs as well.
Although it is convenient linking several IMAP services together does not seem to be that secure.
The agreement that Apple issued IDs would be recognised as AIM screen Names goes back to the start of iChat (version 1) in 2003. That's 10 years.
I had a quick look at the Google Settings and it only list being able to have other IMAP services pick up the mail and nothing about importing other services. (it was only a quick look)
I can see it might be a possible though that the agreement is coming to an end.
10:26 pm Tuesday; November 19, 2013
iMac 2.5Ghz 5i 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad