The term "收率" is kind of confusing anyway. In different books they may have different meaning. That is why some people give a definition before using them to reduce confusion.

But I think your estimate of 0.5% is too low. I would estimate majority of 甘草酸 within 甘草 will dissolve into water after you boiling them (assuming 2 g 甘草 in 200 ml water). It may be not 100 % as 方舟子assumed in his article, but should be close.

I have no data to back this up, just gut feelings based on my experience working in chemical industry.