Instead of a parking garage, the belief is that there will be a large surface lot for day guests.
I find this interesting - I would think, although more expensive, that the garage would make it easier to pack more people in. It would certainly be easier to build a garage now rather than wait until capacity is way up.
To be fair, Epcot does have a big ass Lagoon.
I was going to joke "Hey Google, what's the size of the Lagoon at Epcot?", but the answer is 40 acres. Oh, Internet, always educating us.