The extension adds a Facebook comment section below the video player on YouTube. You can still get to YouTube comments on a video. The extension simply collapses them; they're still there and just a click away if you're interested. A quick scan revealed that most videos do not have Facebook comments, but I found a few that did, and they had a decidedly international flavor.
If you are logged into Facebook, you can comment on a YouTube video with this extension right from the YouTube page, and there is a check box to share your comment on Facebook.