tisthemorning, that's only partially true.
Ike's side b interacts with hurtboxes (the things that can be hurt, usually characters), not hitboxes. Normal projectiles should hit him out of it. PK fire is fairly unique in that it has a hurtbox that ignites if it's damaged as well as if it's hitbox hits something. Basically, Ike's side b is registering a target and finishing the move without him getting hit and having a chance to do that again.
That said, there's a great way to gimp aether, too. You loop a pk thunder up and around his sword when it's spinning, and hit the ike on his way up. You'll hit him out of it horizontally towards the stage and stagespike him.