It doesn't really matter which vector you are looking at, since they are using a tiny constraint in a high dimensional continuous space. There's gotta be an unfathomable amount of vectors you can fit in there. Certainly more than a few billion.
No, yeah, totally. Even assuming binary vectors 2^768 is a ridiculously huge number. The probability of collision even assuming a bad sampling that discards 75% of dimensions is still vanishingly small.
