In US there is no regulation of supplements. So if I buy Resveratrol, and the manufactorer and label clearly state "Resveratrol from japanese knotweed" does it have to contain it and not containing it violate some kind of law?
If the label says "standardized" it means that the contents have extracts in the amount stated on the label. There are companies that self-regulate and actually take legal action against other companies who just pump out pills - when they attempt to put standardized on the label. The downside to this is that the standardized extracts say things like 10X which means the ingredient in question has been concentrated by a factor of 10, it may not guarantee 10mg in each pill, or each drop of liquid.
Many vitamin and supplement pills contain varying amounts of the active ingredient.
To answer your question, it must contain at least some japanese knotweed extract if the label indicates it. Who knows how much?