Under designed may'be, but to blame the material is pure anti Chinese/Taiwanese bullshit
Most of the worlds titanium is mined in Russia and China, however it is the mills that turn it into tubing. The US mills produce a higher grade of titanium. I've seen examples of both US and Chinese tubes first hand and just by peering inside the Chinese tube you can see weird wavy shit going on, which I was told was caused by inconsistencies in the milling process.
How many Moots have you seen breaking like this?
Was the junction between the top tube and seat tube properly mitred, aligned tacked and realigned before welding?
Is the tube wall of adequate thickness to be able to cope with the forces at this junction?
Was the tube of even thickness at this point?
These are just some of the factors that could account for such a failure.