I was really pleased to see this; I read all the WHO literature, including the underlying data, before weaning my baby at 17 weeks. Having looked at all the evidence itself, I decided to go along with what I felt was the best thing for my child.
The WHO research is based on collated data from both developed and developing countries. It is based on the premise that malnutrition is a significant cause of infant death, particularly in the first year of life. As far as I recall, it didn't look at allergies etc. but purely at infant death. To me, it seemed obvious that children in developing countries, where they don't have access to clean water supplies, a range of weaning foods etc, would be much better served by being exclusively breastfed for 6 months.
On the other hand, Western babies, whose feeding utensils are sterilised and anti-bacterialised and whatever else, and who have access to clean water, organic food, etc. are at far less risk of getting gastro infections which kill them, which is what the WHO is trying to prevent.
So I didn't feel like I was forever damaging my child by ignoring the WHO advice and doing what I felt was right, although I was often made to feel that way by other mothers who thought it was their business to criticise my decision. I hope that this will help to stop the savaging that some people get for deciding to wean their babies when they feel the time is right.
(PS I continued to breastfeed along side weaning and I hope that this study isn't misinterpreted by people as saying that breastfeeding is harmful; that's clearly not what it concluded).