Just guessing here: Oyama used a less strong aluminum alloy and/or a welding process that wasn't as strong for price point reasons vs. the Dahon original design. At some point along the reverse engineering-consumer experience timeline, they realized they needed to add the rear fork (chainstay) braces.
See the Dahon (unbraced) design undergoing stress testing here:
https://youtu.be/6iO14EPJ2-U
Possibly,... We'll eventually find out.
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