Well, it doesn't require you to input grab first. There's still something like a two or three frame window at the start of a pivot where you can still successfully input a pivot grab. There's an animation quirk where the first few frames of a pivot grab incorporates the first few frames of that character's pivot animation, thus leading people to mistakenly believe you need to input grab after pivot. But the jist of it is, make your grab and pivot inputs as close to each other as you can.
Don't be content just practicing the timing. Perform Pivot grab during an extended run, from foxtrots, from a walk, from an empty jump, and of course the fastest possible where Bowser hardly moves but a step before the grab initiates. The instant pivot grab can reach even further than Bowser's jab, despite Bowser being physically further away from the opponent. But performing inputs too fast will result in a stutter step reversed standing grab. An impressive tech, but not the one we want.