Absolutely UNTRUE. I was there in April and stacked them. I'd take everyone in my party's tickets (7 or something) and go grab 7 at a time.
The thing is if you grab your first fast pass at, say, 9:30 am it will say come back at, perhaps, 10:30 or later for the ride. Now, put that same ticket in for right away it will spit back a receipt telling you when you qualify for another fast pass (this is NOT the same time as on the actual fast pass ticket, generally after, but it could be before). Then, go back to another fast pass machine at that time and you can stack them.
Also, and this is
of the utmost importance, when you get a fast pass, it will say come back between, say 11:30 am and 12:30 PM. You must understand that it's VERY rare for an agent to deny your ticket as long as you're there after the 11:30. By this, I mean come back at 2, or 4, or 6. They don't care. This is why the touring plans book is so excellent, it tells you all this.
If you're not getting as many fast passes as you can you're doing it wrong, and do not think you need to come during the window, as long as you're there after it begins.
I know the above may sound confusing to somebody unaccustomed to FPs, but it's not, you'll get the hang of it

We were there several days and I used the majority of fast passes after the "end time" of that pass and never got a single look.
Bring a backpack full of frozen water. Food and drink prices are not actually that terrible there, but they can rack up. Also, it's worth reserving a spot at a restaurant shortly after you get there, or even call on the phone (number is on the park color maps that you get for free near the doors). If you wait until supper you will be having a hard time getting a spot.