The reason for this recommendation is that AP has a half-life of roughly 6 hours (according to what I've read here), meaning that the product is still present at about half-the strength 6 hours later.
While you could take AP with another high-carb meal say 3 hours after your 1st meal with AP, it might be easier on your pocket to wait 6 hours. So no you don't have to wait 6 hours explicity.