That advice comes from the R/C community, which has had a lot of problems with LiPo batteries catching fire. If you want pictures and video, google for "lipo fire". Several houses and cars have been destroyed. The damage can be caused by overcharging the battery (perhaps putting the wrong parameters in your charger) or by physically damaging it (like can happen in a crash.)
R/C planes and cars typically eschew the protective circuitry and use the lighter batteries (not those with metal cases) for better performance, but of course they become more fragile and more susceptible to abuse.
In general, the chargers have circuitry to keep the voltage right, and then the batteries (for general rather than R/C use) have circuits to disconnect everything if something goes wrong, but there's always room for trouble. The 18450 batteries that the Magicshine uses have metal cases and protective circuitry, so they're likely to survive most damage (well, not catch fire), and even if they are abused they're not likely to catch fire or explode.
Still, the danger is not zero, and Geomangear is covering their ass here. Occasionally you hear about laptops or cell phones doing this, though they do like to blame aftermarket gear for it (which may or may not be true.)