No its the truth. Oxygen is the root of all corrosion, whether you metal, plastic or human (anti oxidants). The deeper in the ground the less affect oxygen has. Not only that a lot of humous soils are not neutral in Ph and can cause problems that way, so if you did below the level of ground soil your better off.
There are prescribed minimum depths for cables BS7671 and Regulation 522-06-03 where it states:
Buried cables shall be marked by cable covers or a suitable marking tape. Buried conduits and ducts shall be suitably identified. Buried cables, conduits and ducts shall be at a sufficient depth to avoid being damaged by any reasonably foreseeable disturbance of the ground.
Theres a table of depth vs voltage vs area. For residential applications at 240v it is 600mm.
Further more it was covered by NICEIC in Connections Journals 143 and 144 and a Technical Manual topic C5-77. These topics state that a sufficient depth is usually 600mm deep for residential purposes.