Reverse polarity protection for LiPo
Because R2 is connected to the charger side of the FETs, it is exposed to the full charger voltage. A properly designed Li-ion charger should never put out more than 4.2V anyway - so this shouldn't be a problem unless the wrong charger is used, or its voltage limiter fails and produces an abnormally high output voltage.

The 'possible' in "as large as possible" obviously refers to the largest permitted value, not just what is physically possible (which is infinity Ohms or an open circuit - not very useful). I would use the recommended 2k&ohm;, which limits reverse current to ~2mA at -4.2V.


  is R2 designed to protect against reverse polarity protection even if
  it's embedded in a device that drains the battery rather than one
  charging the battery?  The language it uses seems to have an emphasis on
  'charger'.


Draining the battery won't put a negative voltage on it. The protection circuit handles chargers that are incorrectly connected in reverse (either directly or through a device's charger socket), or installing the battery into a device in reverse (which would probably fry the device before you got a chance to charge through it!).