Refer to below.
Not just Hiraishin, but also his reflex speed. While he can move instantly to a point with Hiraishin, his reflex speed allowed him to evade attacks after moving to his intended area. As shown during his battle with Ay, he was able to react to his attack, and utilise Hiraishin to move behind Bee. Hiraishin did not facilitate the way he reacted, his natural speed did.
Understand that reacting isn't physically moving from one point to another with the intention to evade an attack. As far as a movement is in response to an attack, it can be called a reaction. Minato was able to raise his hand, lugged his Kunai upwards against Ay attack, hence he reacted.
Using another example; when Naruto evaded Ay, Ay stated explicitly Naruto was the second person to evade his punch. Ay couldn't have said this if he wasn't making reference to Naruto's reaction speed.
I should also add people constantly assumed Minato was moving with his natural speed, when in fact he was teleporting. Case in point; when Naruto evaded Ay punch, Tsunade suggested Naruto's movement is similar to the Yellow-flash. In a way, she's suggesting Naruto moved instantly just like the Yellow flash. This is highly unlikely considering it's impossible to physically move instantly, and as such it's more of a probability Tsunade assumed Minato was moving with his natural speed hence why he associated it with Naruto's speed.