So... "Assassinate" basically has the intent of "If the assassin gets the drop on his opponent and hits, he does more damage because he's an assassin and trained to strike vital areas when his opponent doesn't see it coming or doesn't have time to react".
But if the assassin is too slow, his stealth is irrelevant as the victim does have time to react as is clearly stated by the fact that after the target's turn has passed, they can take reactions.