I was thrilled when he wrote this. I've long been concerned with the notion of pure CF tanks (brittle, limited heat tolerance, capable of severe reactions with LOX, and tons of other things), and I've long been a big supporter of titanium in rocketry, which is what I believe he's referring to. Aerospace titanium alloys have superb specific strengths, retain strength at high temperatures (e.g. reentry), suffer little from fatigue, etc - they're perfect for a long-term-reusable spacecraft. As an added bonus, the rainbow-coloured oxidation they'll get on reentry will be stunning
Titanium is expensive and difficult to work - no question about that. But for a craft that they plan to have reused a thousand times, you'll amortize that investment quickly.
The Shuttle was initially supposed to be titanium, but budget cutbacks during development ruled that out. Had it been titanium, the TPS would have been much simpler, and allowed for far cheaper turnaround between flights and far greater reliability. With titanium you can let the frame run hot; you can't do this with alumium. They wouldn't have needed the problematic ceramic tile system, they could have gone with a simpler TPS.
Again, Musk hasn't confirmed titanium, but it's the obvious choice for a metal that's heavier than alumium.