My reply wasn't meant to be disparaging in any way. It was to point out that your assertion that unthreaded discussions tend to be more focused and tight compared to threaded discussions isn't necessarily true.
Regarding the example I cited. it's an unthreaded discussion that has been going on for several years and, if you bother to read through it, it's a mixture of troll comments along with some informative comments/discussion. Had it been a properly threaded discussion, it would have been much easier to follow the informative thread branches and filter out the uninformative ones (where the classification of what falls under one category or the other is up to the person reading it).
But, because it's not threaded, you're effectively forced to wade through all the troll posts to find the informative comments (or miss the informative comments entirely if you're not willing to do that). The lack of threading did not change the "social aspect" of the discussion. It would have been the same either way given the topic, but one way of rendering it would have made it much easier to follow (and possibly spawned more informative discusson about the topic compared to what actually took place).
Getting back to your points:
> the very first comment is a message to the requestor telling them so. [...] after the message explaining that GitHub is not the correct place to make a pull request and that it will not be moving forward
If your assertion about unthreaded discussions was true, then it should have ended soon after that comment (maybe after a couple of comments from the person who submitted the pull request after Linus' comment). That obviously wasn't the case and nothing kept it in check.
> the unthreaded approach can help keep the discussion focused
Based on what I've read online over the years (both threaded and non-threaded discussions), it doesn't seem to be the case. The only way I think this could be settled would be to come up with a representative sample of discussions under both formats and how well they stay on the original topic compared to each other. It would be interesting to see the results of such an analysis.