I do not give exact quantities, other than 1/3 to 1/2 bread flour, because the rest can be any other flour or mix of flours you choose.
But you need that much hard wheat (durum) flour, because it contains enough gluten to make the bread rise. Rye, oats and barley contain no gluten (yeast will not make them rise) and whole wheat and other grains are low too, so you HAVE to have enough white flour to carry upwards the rest of the grains and seeds you use.
Enjoy the video, and make some. It is great bread!