It's not an AT, it's just punishing Marth, just like one of the people above me said.
An AT is the result of being able to exploit a programming flaw for your own benefit, in most cases (like DACUS)... It can also be the proper application of game physics in a way the programmers didn't intend it to be used (like IC's infinites or Kirby's kirbicide>footstool).
What you described was you taking the amount of time between Marth's 3rd and 4th DB attacks and hitting with a quick attack of your own... All characters with an OoS option can do this, for example: Kirby can shieldgrab Marth, or ftilt, or utilt, or dtilt...