Is this MOSFET upside down?
It is correct. You normally see PMOS connected like this to act as a reverse-polarity &quot;diode&quot;.



simulate this circuit &ndash; Schematic created using CircuitLab
It has much less voltage drop than an actual diode and will protect currents from flowing the wrong way when the voltage is connected between the battery terminals are connected in reverse.
It will NOT protect against the load pushing current back towards the source like a real diode will. That requires additional circuitry to accompany the PMOS.

When power is connected the correct way, the parasitic-diode gets current flow started which produces a voltage drop across the load, and this voltage drop appears across the gate-source which turns the MOSFET on. At this point the current flows through the source-drain of the MOSFET rather than the parasitic diode which results in low voltage drop.
When power is connected in reverse, current can't flow through the parasitic diode, and with no current flowing through the load, the voltage drop across the load is zero which also means the voltage drop across gate-source is zero so the PMOS does not turn on to conduct anything between source-drain.
At this point you may notice that the voltage drop across the load is what triggers the PMOS to turn on, which means that if the load develops a voltage that tries to push current in reverse, the gate-source will be biased in the correct way to turn the PMOS on and flow through the source-drain back towards, bypassing the diode which would otherwise block it. This is why it can't stop the load from pushing current back towards the source.

The 100K there is to allow the PMOS to do its job. In a simple, normal, reverse-polarity PMOS circuit the gate would be directly connected to GND, which obviously doesn't work here because it would short out Vbus.
In this case, a twist has been thrown in where Vbus can pull the gate HI turning the PMOS off which prevents Vbus from back-charging the battery (something which I mentioned earlier a regular, simple PMOS reverse-polarity circuit can't do). That's something new to me. I'll have to remember it.