SP, I think the crucial point here is that the effect was detectable in each month. It would have been easy to spot if the effect had been strong in months 1-4 and then disappeared after that.
I really wish they had published the actual figures for each month so we could see for ourselves the size of the effect at 6 months - that would be very interesting.
But as you point out, after 6 months, few babies are exclusively bf and you would want to be cautious about extrapolating from, say, the 7 month results to the population at large, as they may be a special group.
Incidentally, WHO didn't use this study to recommend 6 month weaning age - it's only been published recently, ie a long time after the WHO recommendations.
I've read the report of the expert consultation and there is absolutely no chance that the 6 month advice was made universal in order not to "patronise" some countries. The advice was based on evidence from both the developed and developing world, and was biologically, not socially based. It's the same worldwide because biologically, babies are the same worldwide.
I disagree with you about being sceptical of its results - it's the biggest study of this kind so far and it has a very strong design. 2 other similar, but smaller, studies have reported similar results (one from Spain and one from the US if I recall correctly).
Studies which didn't show an effect did so because they were too small to prove an effect reliably, rather than convincingly demonstrating no difference.
It also fits with what is known about the development of gut immunity.
So I'm pretty convinced, although we (parents) still need lots more info, especially about individual variation in risk.