I'd assume it's only YouTube, the usage message has this text [1]:

    Download a video from youtube

The beauty of youtube-dl is that it is so universal, supporting a great number of sites, even the simplest case of giving it a direct link to a video file and it will essentially wget it, and process according to your config.

[1] https://github.com/kkdai/youtube/blob/00a2aefb29c4fbf4fc3bf3...