I just assumed the current place was a way of honeypotting dissenting organised accounts. It wouldn't be the first time place had mod/administrator drama. Granted last time they were just overwriting pixel data, but logging account id and timestamps in addition to a hex code is pretty trivial extra code.
I fail to see the goal of "finding dissenters". Reddit wants users. They aren't going to ban users for placing pixels. Besides, they already know the worst dissenters because they write fuck spez in their comments.
They can flag the accounts and shadowban based on metrics. Keeps them in total user count, might drive more signups too afterwards. They know who writes "fuck spez" in comments, but they don't know which accounts are run by single people or coordinated groups. Place potentially provides some of that information.
Reddit wants users who engage authentically with content, generate interesting content, and interact with ads. High-intent users, not people who coordinate to post "fuck spez" and bad-mouth Reddit.
Whether they're forward planning enough to realise that is another issue altogether.