I am not sure why that person is worried that "The MOSFET at the bottom is an IRF part that's .. always on." -- this looks like a fairly standard reverse polari

I am not sure why that person is worried that "The MOSFET at the bottom is an IRF part that's .. always on." -- this looks like a fairly standard reverse polarity protection circuit, and it is extremely power efficient. The only current is Vgs leakage, and it is usually very small. I cannot read the part number off the photo because it is too fuzzy for me, but if we take a random IRF mosfet in this package, like IRFL014, it's gate to source leakage is 100 nA, or 0.0001mA. I would worry much more about the "op-amp bits" and also whatever regulator makes +5V, those are the parts that usually have non-trivial quiescent current.