That tutorial isn't quite correct. What he's demonstrating there is actually negative priority, where the attack hitbox is inside the collision hitbox, thus giving the attack a disadvantage.
Infinite priority isn't necessarily where an attack hitbox overlaps with a collision hitbox, but more so where there's no collision hitbox anywhere near the attack hitbox. As many have said, priority is determined by the hitboxes, so for an attack where you want it to "win" against other attacks (such as a heavy attack vs. a light attack), you'd want to extend the red hitbox out a fair bit, but not so much that no attack can counter it.