The battens are the secret sauce. One of the biggest struggles in light wind is pre-inflation of the kite, the battens solve this issue. They also provide much needed stability in light winds.
A 21M Sonic3 built with battens would also be a ton more fun than a 21m Soul. Better yet, a 23 since the depower is so good. It is the ticket for fun riding in 10 to 15 knots on a big, fast twin-tip.
Perhaps, Flysurfer may not see the need for such a kite as many riders have moved to hydrofoil, but not everyone and for snowkiting a 21 and/or 23m would be the bomb on the light, deep powder days. If FS designers are unconvinced, grab a fast twin-tip at the next race and try the 23M VMG in 10 knots, it will put a smile on your face. Much more fun than the Soul 21m in similar conditions. People are still buying and using the 21m Soul, but almost all of them are using the big Soul for free-riding, not hydrofoiliing. You really don't want or need that big of a kite to foil in lightwind. Most of my friends use a 15 and a couple very big guys foil with an 18, but I have never seen anyone even try the 21 with the hydrofoil.