A rhizome is functionally similar to a root, but not exactly the same thing. It serves to anchor the plant and it takes up some nutrients, but it should not be buried. Anubias anchor on surfaces like a lichen or tree-growing fern. The rhizome will rot if buried and the plant will die.
The proper way to plant those things is to tie them on to a rock, piece of driftwood, or other tank ornament with thread or fishing line. They will over time attach themselves to the object, and the thread can be removed (or cotton thread will rot).