Shelters that are designed to be buried, even partially, are designed (or should be designed) to withstand different stresses than an above ground shelter. A buried shelter needs to withstand stresses like ground freezing and shifts in the water table, while an above ground shelter would put an emphasis on wind and projectile protection. Installing a shelter in a way that the manufacturer did not intend could put your safety in jeopordy when you really need it. Could have a problem with an insurance claim, too. So, unless this "someone" is an engineer and knows the specs on this particular shelter, I would stick with what the manufacturer recommends. If you're going to go to the trouble of purchasing a tornado shelter, and you want it above ground, then get a good above ground tornado shelter. But all things being equal, I would much rather be below ground if a tornado hits.