At the time, Symposium requires that all video submissions are uploaded through YouTube. This is because YouTube meets all accessibility requirements (automatic closed captioning, etc.) and automatically compresses content well for viewers with low bandwidth.
If presenters are concerned about publicly posting their videos on the internet, they can easily set the YouTube visibility settings on their video to "Unlisted" to ensure that only people with a link will be able to view their video. Instructions to set the visibility to unlisted are provided within the Symposium presentation submission form, and may also be found here.