This is correct. The two drivers are "rate" and "volume". "Rate" refers to what the customer actually pays, not the rack rate. Growth has recently been driven by an increase in the percentage of guests paying the rack rate.
You're correct when it comes to people in the hardcore Disney community because we know that waiting for certain times of the year will yield discounts. Most guests book very far in advance and simply don't do their research. Discounts will always be used to some extent, so they're not looking to "do away with discounting."
A cruise ship is a good example. It's better to fill 90% of the rooms at full price and then offer steep discounts for the remaining 10%. If you're offering a 20% discount the entire year, you're missing out on a lot of marginal revenue because that 90% of guests were willing to pay full price so you were essentially giving them a discount for no reason.
Another important fact is that Disney can't adjust expenses to match demand like a restaurant. If a restaurant is only half full, it only needs half the wait staff and will incur half the food costs. Conversely, Disney can't adjust the number of rooms and buildings it's paying to operate. Small adjustments can be made to labor levels, but it's much more efficient to get those rooms filled, even if it requires a discount.