As far as I'm concerned, one of the main points (if not the main point) of having threaded comments is to keep things more organized, separating different discussions in their own threads.
I actually needed to write a few single-line small comments
I understand that you thought your comments could be "independent" (not part of a thread), but in the end they're talking about something, and that something can become a discussion (AKA a thread): if someone replies to that comment and start discussing about what you wrote, well, that's a thread. Maybe it wasn't your original intention when you wrote that comment, but what was intended to be a "solo comment" now has became a thread.
And what should be done in that case? Convert the non-threaded comments to a thread? Or keep the comments unthreaded (but now they're talking about the same subject, so it'd be better if they were part of a thread)? I believe that it's simpler and better to just make all comments threaded by default.
I believe that mixing two types of comments (threaded and non-threaded) would be an unnecessary complication: it would not only make the UI more cluttered (threads are collapsed by default, how should the non-threaded comments be displayed?), but also - I guess - it would make the implementation unnecessarily more complicated (consider the case I mentioned above - and perhaps many more corner cases that would happen).