We must always be careful replicating wild habitats too authentically. While it is true (the videos show it) that the wild Betta splendens lives in swamps and similar watercourses, and that these obviously do not have gravel or sand but soil/mud substrates, covered with leaves and thick with plants...there are reasons not to copy this exactly.
Soil is the big issue. In an aquarium, we are immediately dealing with a very "un-natural" environment. Biological and chemical relationships that occur in the wild habitat can have disastrous consequences in a closed biological system. What is easily dealt with in nature becomes a death trap in an aquarium. There is really no benefit to soil; the only plant benefit is the CO2 and ammonia from the organics in the soil decomposing; this usually will continue for up to a year, at which point the soil is basically exhausted and of no further benefit. A sand substrate works the opposite, slowly building up into a beneficial substrate over the first several months, after which it will continue to benefit almost indefinitely.
Those advocating soil tanks usually recommend a dry start method, and waiting six months before any fish are added. This avoids the ammonia poisoning problem with new soil substrates. Depending upon the organic level in the soil, ammonia can wipe out the fish within hours.
Most of us would not want the mess of a soil substrate on its own, authentic though it may be. So sand is the preferred alternative. It may not be strictly "authentic," but it is clean and safe by comparison. Dried leaves, wood, plants can be added.