The youtube video player does not support 21:9 resolutions. People with 21:9 monitors will always get black bars around video uploaded at 21:9 - because youtube will add horizontal black bars to make the 21:9 video 16:9, and then the player will add vertical black bars to make the now-16:9 video fit on a 21:9 monitor. There is no way to get around this until youtube updates its video player for native 21:9 support.
Until that happens, though, the method I suggested to you is the best option at this point - it preserves all the detail of the original video, results in correct playback for people with 16:9 monitors, and does not require any extra editing steps on the uploader's part.