According to Star Trek technical consultant Michael Okuda and creator of the Heisenberg compensator, “It works very well. Thank you.”
Earlier in the writing of the episode, the Planet killer was more of a moving fortress, replete with additional weapons that had been disabled by time and countless counterattacks. By the time Enterprise encountered it, it was largely a wreck, with fundamentally only the main weapon remaining.
Norman Spinrad was displeased with the model used for the planet killer. As he told Allan Asherman in The Star Trek Interview Book [page number? • edit], he envisioned the planet killer bristling with all sorts of evil-looking weapons.
And…
The Ships of the Line (pg. 52) book states that the Planet Killer once had been the monster Norman Spinrad envisioned, boasting a “complete arsenal of weaponry”, but centuries of combat had taken their toll so that, by the time the Constellation encountered it, only the main weapon remained.
So, if you want to take the in-universe explanations (or retcons) then such an attack wouldn’t have been possible before the accumulated damage of countless desperate attempts hadn’t largely ground the device down.
Golf clap. 
You’re on a roll today with great comebacks.