Sorry you had a bad experience many years ago. I understand disliking the company for any number of reasons.
Suggesting everyone leaving the company under normal circumstances must hire an attorney is at best poor advice.
Your experience and information isn't analogous to people leaving today because everything is different from process to systems.
AIG doesnt control access to your 401(k) and they couldn't unless they were the plan administrators. They may have been at some point but not in many decades.
For most people the pension has been converted too. Punch a button to start the process.
For us little people it makes more sense to wait a few weeks rather that incur unnecessary attorney fees.
If you have an employment contract or leave under dispute it may be a different story to get help negotiating.
As to why a 4 week difference? Don't know. Payout for everyone used to be in March. They moved it up for employees.
Retiree payout is probably outside the system/process for employees.
I know plenty of folks who have retired in the past couple of years and had no issues.
Or no more issues than employees do when trying to get info through shared services.