This circuit looks odd the first time you see it. When a MOSFET (P or N-channel) is on it conducts in both directions.
But, in order for a discrete MOSFET to block current, the body diode must be reverse-biased. The circuits you linked to are correct. When the power supply is connected properly, the MOSFET will be held on and will conduct with low resistance.
If the MOSFET in one of those circuits were to be reversed, it would still work in the forward direction. But then in reverse polarity, even though the MOSFET would be biased "off" by gate voltage, the body diode would be forward biased. So the MOSFET would conduct via the body diode and the main purpose of the circuit would be defeated.