But, that doesn't explain why he never mentions titanium, which is the other half of the question. If you even mention aluminum, the titanium question is clearly more relevant.

And, if these things are to be re-used, up-front material cost is less relevant than value. Titanium maybe doesn't get stronger at cryo temperatures, but is it still stronger than steel at such a temperatures? I couldn't find numbers. If it is, the dry weight of the vehicle could be much less, with large follow-on benefits.