What is often overlooked or forgotten is that it is Stan "the Man" Uris who actually initiates the promise. He is the one that goes and gets the coke bottle they cut their palms with. This is because Stan is already thinking ahead, and I suspect already knew he would not return. I think Stan knew already on some level that he couldn't face it again, but was too ethical and moral not to ensure that the others would be back. He knew, even back then, the implications of what he saw.
More to the point, Stan's very ordered mind didn't forget everything. Remember, his wife talks about the fact that he recognized Bill's name as an author of a book and clearly remembered and reacted to what he read. The others had seen that book and didn't make the connection until after the calls from Hanlon brought them back. Stan Uris may not have made it back to Derry, but without him none of them would have made the promise in the first place.