Throwing some black worms in. Planting and letting ditrius rot and clutter the substrate. Then adding external filtration and doing water changes. I wouldn't say that was a functioning dbs.
Put very simply, even without plants, snails and worms, think of the deep sand bed as a huge bio-filter.
Many have come to believe that beneficial bacteria only lives in the filter. In fact, in the established aquarium, there is far more biology in the substrate. Deep sand just extends the platform to also accommodate anaerobic bacteria in it's deeper layers. The key is to NOT disturb the sand or disturb as little as possible.
Deep sand, like any other bio-filter, does not eliminate a need for some mechanical filtration, water circulation, and water changes. HOWEVER, leveraging [more] biology allows us to have a healthier tank with less filtration and lower volume (or less frequent) water changes.
Taking it to the next level, deep sand, living plants, MTS/worms, etc. leads us to a nearly self sustaining biotope. Consider...
Aquabella.com
Aquaripure.com
They tell us that with only modest mechanical filtration and the right bio-filtration, we can dramatically limit water changes while maintaining healthy tanks with crystal clear water.
Oh you can run canisters + hobs and change out 50% of the water weekly - that's one way....but just maybe it's not the only way and may not be the best way.