They only recommend it for container-grown plants in certain proprietary growth mediums.

Unlike most other plants, grasses metabolise silica, and a lot of it. Sand, silt and clay are three of the four main components of soil, the fourth being organic matter. Sand is largely silica (but possibly not so easily available for metabolism); clay is hydrated silicate rock, (and probably therefore provides silica most easily available for metabolism, in general); silt is intermediate between sand and clay. So if you are are growing bamboo in a balanced soil in the ground, or in a soil-based growth medium, I think it has plenty of silica available.