I think that note is referring to running the "speed pinion" on 6s, in a off-road / high drag surface. This WILL lead to huge heat in the motor and ESC, and could burn them out.
High drag surfaces, like sand, loose dirt, or grass needs lower gearing. Higher voltages needs lower gearing. So yeah, the warning makes sense to me.
Born again RC'er.
Arrma Kraton, Arrma Typhon, ECX Torment 2wd, Turnigy 1/16 Mini-Trooper, Losi XX-CR