I don't think that's what this is about - it isn't so much that they remember this as the first time they 'met', it's more that they both remember the encounter. They may or may not have figured out the details in later years - I don't think it ever says as much in the captions - but the actual experience is something they both remember. Clark remembers going to fetch the ball with his friend and coming across the sad boy in the car, Bruce remembers BEING the sad boy in the car, and seeing the two boys stop and look at him. It's one of those brief encounters that has little actual impact, but which you remember vividly nonetheless.