[graemep]

He is clearly wrong on multiple counts.

"If the source code originally used AGPL, isn’t it still contractually obligated to ensure those rights to its users, including any new source code added to it?"

The answer is clearly no. The GPL is not a contract NOR does it contain any such obligation.

"It doesn’t matter that you own the copyright on the software, you can’t revoke the rights on the code that was licensed under its terms."

You cannot revoke the license, but if you hold the copyright to code you can dual license or license modifications under a different license.

"This is a very bad post, and I'm surprised it made the front page. "

An illustration of how little copyright law is understood by developers. I have seen lots similar things in the past. A bit unfair to pick on old stuff that the author probably knows better about by now, but this always stood out as a good example: https://jacobian.org/2009/jul/13/gpl-questions/ - twenty questions, the answers to most of which are fairly obvious or easy to find out.