I don't see a problem with the structure... It's like a bf but with the further rather stylish reinforcing thin double tubing brace. Gives very much fore-aft bracing while retaining a low step-over height. Braces the bottom bracket shell against torsional forces of pedaling. Braces the head tube against same. I think it is very carefully thought out.

There are plenty conventional diamond mini-velos; good to have a very stylish alternative that is IMHO also very stiff.