The reason "over 800 bar of hot oxygen-rich gas" is a high hurdle to break is the propensity for a metal to rust and become useless is proportional to the pressure (concentration) of the oxidizer (oxygen) it's surrounded in and how hot it is. So this metal is able to withstand 800 bar (1 bar ~=1 atmosphere), or 4 thousand times as much oxygen as sea level, at quite elevated temperatures. This is a hellish environment to not just immediately combust in.

And note that at 800 bars of hot oxygen, metal doesn't so much rust as it burns with a bright flame. The environment in the Raptor oxy-rich preburner is much worse than the typical oxyacetylene torch that is used to cut steel.