Does 'Sakuretsu Armor' negate the attack?

No, but it does stop it. 'Negate' is a very specific game term, which has other meanings and significance. All Sakuretsu Armor does is make it impossible to continue the attack step by removing the attacking monster - the Damage Step will not be entered, so the defender will not flip if face-down, and no damage will be calculated. However it is still not accurate to say it 'negated' the attack, which other cards like Necro Gardna actually do.