It's important to remember that a 300-minute wait time, posted less than 5 hours into the day, is an educated guess, at best. This was not calculated by a red card that traveled 5 hours through the line, because the park hadn't been open that long. Even the red card calculations are a somewhat inaccurate predictor, as they only tell the computer what the wait was for that person when they entered the queue, however many minutes ago.
Disney is pretty talented at providing wait time estimates... but they are just computer calculations and estimates of current conditions based on past conditions. They are not a fully reliable predictor of what you might experience when you enter the line. Most of us have had the experience of seeing a really short wait time for Peter Pan (or other attraction), only to arrive at the same time as countless other people who saw the same really short wait time. While I think Disney has good data on most rides that have been in existence for a long time, I don't have a high level of confidence in their ability to guesstimate wait times on a brand-new attraction. (I know, they can somewhat rely on TOT data... but I doubt TOT provided much data in the 300 minute range for them to analyze.)
Having said that... those photos look like some *long* lines. I'd be interested in hearing from someone who actually waited in line and timed their wait.