They did not, at least not until much later, many years after AOL waned in popularity -- possibly in part due to lack of money to pay enough engineers.
Initially AOL's was done (GUI and non-GUI functionality) via their client API called FDO and sent assembled packets out like everything else. AIM on the other hand was a classic simple GUI app using a different protocol and used none of AOL's FDO stuff.
Much later AOL used some OSCAR (AIM) libraries to piggy back the AOL client on-to AIM for buddies and IMs, but even so these were not a part of the exploits that kept popping up for AIM, and long after AIM's drop in popularity as well.
You are confused, AOL and AIM are two totally different pieces of software, which use different protocols and serve different purposes. The exploit in question has literally nothing to do with AOL, it's for AOL Instant Messenger. MSN never tried to access AOL's service network, but certainly AIM's.
Initially AOL's was done (GUI and non-GUI functionality) via their client API called FDO and sent assembled packets out like everything else. AIM on the other hand was a classic simple GUI app using a different protocol and used none of AOL's FDO stuff.
Much later AOL used some OSCAR (AIM) libraries to piggy back the AOL client on-to AIM for buddies and IMs, but even so these were not a part of the exploits that kept popping up for AIM, and long after AIM's drop in popularity as well.