ok i guess i'll ask the obvious.. why such a shallow substrate?
Why not? There's no reason not to have a shallow substrate in the front of the tank per se or at all really. You can still very easily grow plants in 1 inch of substrate. You can use more to create a slope or deeper soil bed or less to create an accent but plants will still grow. Look up some of the sand wabi kusa setups. A lot of them use less than an inch of sand and are still beautiful.