Quite possibly he did mean it at the time. However, since the announcement that it would be going to the Gates Foundation, uh, Bill Gates' reputation has had a rough time. Also, it was the Bill & Melinda Gates Foundation at the time, and they (and their money and charitable endeavors) have since split, so if it was Melinda Gates' instincts and skills on charitable giving that he was counting on when he made the initial announcement, that might be a reason for pulling back.