It's even more confusing as the answer is different depending on whether you'll connect the Ethernet Shield to an Uno or Mega.
Becareful... this isn't the Ethernet Shield. It is the "Ethernet Arduino." It does not connect to a Uno or Mega. Its Standalone.
Each of the 14 digital pins on the Ethernet board can be used as an input or output ...
Which is correct?
Technically they are both correct. The generic I/O section was probably copied and pasted from the Uno board description. If you aren't doing any Ethernet operations, you might be able to use pins 10,11,12, and 13. However, it would be advisable not to since they are connected to something else on this particular board.