I think they remember stimulus and situations more than a person or another horse. Once you start riding and working a horse, I'm betting it will remember you more than just walking up and saying "howdy". As said by goldilockz , that horse remembered the cue. It didn't see the old owner from across the field and come running out of love and fond memories.
I have gone to visit my first born several times. My feelings are always a bit hurt when she doesn't seem to remember me. In reality, I'm glad she doesn't have that capacity. It would make the world worse for horses than it already is if they remembered being taken from their mothers at young ages and being hauled from owner to owner as so many are.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France