Well the mission has a few other minor problems, like 'where the heck is everything warping in from'?
That, and essentially it forces you to use a DT wall because you can't produce the other units fast enough to counter the waves (at least on Hard).
On Hard, I managed to get 2624 kills, still short of the 2750 needed for the achievement, despite multiple save reloads. Finally I reverted to Normal and only then managed to scrape by (4 kills over the achievement threshold).
Biggest problem with the end of that mission is the waves become so large that it is impossible to effectively counter anything, and my framerate plummeted to 5 FPS. I stacked a mixed force of 60 ships, Mothership, Carriers, Phoenixs, and Void Rays, all with maxed upgrades and watched it get slammed by enormous waves of Zerg fliers. And I discovered that the computer cheats at the end - even without Overlords to detect under the cloak, the Zerg fliers were targeting the carriers themselves.
2254 kills on Normal was enough.
As for the Overmind retcon, that irritated me too. The Starcraft documentation explicitly contradicts the explanations in Starcraft 2 so far as the Zerg's nature.