Just saw the video and this is quoted from the description given by the creators of the video themselves: "Since youve always wanted to learn about Celtic Warriors, you may as well learn with the best of the worst. Also our Salmon Leap is accurate because we say so."

Notice the underlined part. As for the historical accuracy, let's say I believe you should know better than believe a clearly amateur video made (most probably) for fun. Just use common logic: Would you try a "salmon leap" against an enemy in the first row, thus turning your back to who-knows how many other soldiers in the hind lines?

Maion