"Even though 80 percent of the capacity remains when a car battery dips to around 10.5 volts, the battery is considered to be fully discharged because taking the cycle any deeper will cause irreversible damage the plates through excessive sulfation.

While normal sulfation is reversible, excessively draining a battery, or leaving it in a state of discharge, will allow the soft lead sulfate to crystallize. At that point, charging the battery will still cause some of the sulfation to reverse, but any crystallized lead sulfate will remain on the plates. This sulfate cannot, under normal circumstances, return to a solution in the electrolyte, which permanently reduces the available output of the battery.

The other detrimental effect of allowing crystallized lead sulfate to form is that it effectively shortens the lifespan of the battery in an empirically measurable way. If too much of this crystallization is allowed to occur, the battery will no longer be able to provide enough amperage to start the engine, and it will have to be replaced."

...and, when enough of those lead sulfate crystals settle in the bottom of the battery, they create a short across the plates.

 

He's just being a smartass. Bike isn't an M8.