I'm not sure who you are quoting. - I looked at both your refered threads and could see no STi Japan original source documents. All I could see were some comments from a North American component reseller.
If anyone believes any cast piston is as strong (or expensive!) as a forged equivalent, then one simply does not understand basic metalurgy.
From an application perspective, the choice is more complex than that. However, the basic strength issue is not debatable. Fact is, the U.S. spec Sti uses the high volume, production line Forester short engine. Not the handbuilt short used internationally. - Why? - Simple! - Cost!
An international spec STi costs over $40,000 U.S. A U.S. car $33,000. The short engine assembly represents a large part of that difference.