I think for the fire to start, there has to be titanium fragments, not smooth, solid titanium
But the same report also notes:
"Nitrogen tetroxide with a water content in excess of 2.5 percent can accelerate titanium intergranular corrosion to the point of violent reaction."
I think the assumption is that by the time a RUD starts you no longer care about whether the various parts of your fuel system are going to explode, since the fuel is a far larger concern and it's going to go up anyways.
There’s really no point in designing additional failsafes into a launch vehicle when it disintegrates which can happen for a variety of reasons beyond anyone’s control — RUD is really just an euphemism for an explosion.
It doesn't mean there are no failsafes against RUDs or to protect the crew and equipment in case a RUD occurs.