: So the PoA taking off requires a ridiculous amount of rocket fuel. Physics
: fail. Not canon.

It needs a lot of thrust and that requires a significant power supply. It does not mean it requires rocket fuel in the normal sense. It could be a fusion reactor or some quantum-level stuff (If the UNSC has harnessed Slipspace tech and AIs, quantum effects must be something the UNSC can deal with.)

I doubt it's a supply of liquid oxygen like modern-day rockets.

Though that technology is still in use in the Halo universe. The ICBMs used on the multiplayer map Standoff still use liquid oxygen as a fuel supply.