I don't think this is true. First off..I don't think there is going to be a strong correlation on harry Potter and people at Disney. Yes, I think you will see a big surge in people at Universal/IoA, but I think while it will draw some people from Disney, it will also draw MORE people to Orlando, which will boost the number of people going to Disney. (Few people will spend an entire week at Universal.)
As far as that DAY. Well, some people will want to be at Universal for opening day....but others will want to avoid it like the plague. I would suspect a slightly lower # of people at Disney than typical for June 18th...but I doubt it will be significant enough that you would notice it versus June 17th or June 19th.