As the holes are on top of the butterfly valve I think the issue is when the valve is closed. There's no air flowing by "pulling" fuel/air mixture out of the holes. Instead there's a flow through the idle circuit. This will probably have the opposite effect meaning it will suck air through the holes making the idle leaner. The smaller and the less holes the smaller this effect. Maybe the issue of the quote is the wording and our understanding as it does not say 'richer' and 'leaner' but 'richer' and 'weaker'. However I still don't completely get it
