Just because something is available to view online does not mean you can do anything you want with it. Most content is automatically protected by copyright. You can use it in ways that would otherwise by illegal only if you are explicitly granted permission to do so.

Specifically, Stack Overflow licenses any content you contribute under the CC-BY-SA 4.0 (older content is covered by other licenses that I omit for simplicity). If you read the license you will note two restrictions: attribution and "share-alike". So if you take someone's answer, including the code snippets, and include it in something you make, even if you change it to an extent, you have to attribute it to the original source and you have to share it with the same license. You could theoretically mirror the entire SO site's content, as long as you used the same licenses for all of it.

So far AI companies have simply scraped everything and argued that they don't have to respect the original license. They argue that it is "fair use" because AI is "transformative use". If you look at the historical usage of "transformative use" in copyright cases, their case is kind of bullshit actually. But regardless of whether it will hold up in court (and whether it should hold up in court), the reality is that AI companies are going to use everybody's content in ways that they have not been given permission to do so.

So for now it doesn't matter whether our content is centralized or federated. It doesn't matter whether SO has a deal with OpeanAI or not. SO content was almost certainly already used for ChatGPT. If you split it into 100s of small sites on the fediverse it would still be part of ChatGPT. As long as it's easy to access, they will use it. Allegedly they also use torrents for input data so even if it's not publicly viewable it's not safe. If/when AI data sourcing is regulated and the "transformative use" argument fails in court and if the fines are big enough for the regulation to actually work, then sure the situation described in the OP will matter. But we'll have to see if that ever happens. I'm not holding my breath, honestly.