You are overestimating your 'rush' and using the extreme ride on the entire resort as an example.
900 people an hour can ride TSMM. So in a normal 10 hour day, that is 9,000 people. But the first FP- doesn't start until an hour after the park opens. And how many FP- went out an hour? Were 50% FP? Probably less. Lets be generous and say that is 33%. That means there are 3,000 FP- for an entire day....300/hour. So for FP- to get out to noon, you only need 600 pulled. There are 25,000 or so folks a day at DHS. That is only 2.4% of Guests for the day. Further....how many of those first 600 FP go to people really 'in the know?' and how many got to folks that just like toy story and wandered up 20 minutes after RD and found a 60 minute line? A Bunch.
It is not the 'right' of everyone who walks down hollywood blvd to ride TSMM. In fact, it is mathematically impossible for any more than 40% of the average gate to get to ride it on a normal day.
Be our guest has a popularity and capacity issue as well. What do they have in there? 100 tables? So you only need about 300 'in the know people' to eat up all the reservations during dinner for 45,000 guests. If each party is 4 people that is only .8% of the park guests that get to eat there.
So to answer you question, how does 'everyone know'. They don't. A very small, and on this board, vocal, minority are 'in the know.'