AIM says that the login port is 5190 in most articles.
Some like the one you have come across point out several closely related numbers.
In fact the AIM servers will allow a login on almost any port (there are 65535 in total).
We suggest port 443 as it is used fro Secure mail in some apps and some Secure sites when Web browsing. This means in most external firewalls the port will be open. In the XP and above firewall and the Leopard firewall you have to set the Apps to allow ports which is different but it will allow any port iChat is using in this case.
In some modems or routers that have to use Port Forward some additional features such as stating the protocol for the data means that port 5190 is forwarded on both the UDP and TCP Protocols (And historically Forwarding of one port is not done twice - normally to different IPs (computers) so some have problems with this.)
Therefore the recommendation to use a different port for the TCP login.
443 is also below the threshold at which NAT port start in a modem or router easing problems around this issue.
9:58 PM Saturday; January 19, 2008