Here's the quote in context:
And just to keep this on topic, I wonder if the softness will reduce the hours of The American Adventure, at which Mark Twain is one of the main characters?
Lines don't form linearly, but let's say they do. Let's say the attraction has 1400 pph and 700 people line up for it, and another 700 people get Fastpasses. Those 700 are distributed over the course of the hour (by the use of divergent FastPass return times), they don't all get on at once.
Nope. Because the 900 people won't all be given the same FP return time as the time you are waiting in line for. They'll be distributed throughout the day (or the hour, in your case) due to diverging FP return windows. In a Pre-FP era, those 900 people would be in line with the 700. And, if you're going in the middle of the day, another 500 or so people would be in line in addition to that (again, picking numbers for illustration). So now you have 2100 people in line. Now, because of FP+, 500 of those people in line can't get FP return times during that hour, so they get them later in the day, at less crowded times. So now your middle-of-the day standby line is only 1600, much less than what it was before FP. So you only have to wait a little more than an hour for your attraction instead of a full 90 minutes..
Except before FP+, all those people would be in the line, making the line *longer*. So the line may appear to move faster, but in actuality, the line is longer, so it takes a similar amount of time.