A de Forest Research memo (dated September 15, 1967) actually covered this, but I believe the episode was being hastily rewritten by Roddenberry and Coon throughout filming, so they may not have been able to implement the change.
Also, the dates I have indicate the episode was filmed September 12-20, 1967, so the scene may have already gone before the cameras by the time the de Forest memo arrived.
de Forest Research wrote:
six million who died in your First World War, the eleven million in the Second, the thirty-seven million in your Third – In WWI there were 8.5 million deaths; in WWII, 30,538,000. 37 million deaths for WWIII seems conservative; suggest 260 million.
The fact that the previous two de Forest memos (dated May 10, 1967 and September 7, 1967) didn't address this suggests that this dialogue was a last minute change. That's also supported by the fact that the episode brings up World War III, a topic which NBC had specifically asked Roddenberry to avoid previously (on May 17, 1966, in reference to a revision of 'The Corbomite Maneuver'):
Stan Robertson wrote:
As we discussed, a suggestion would be that on Page 48 of this draft, it be made pointedly clear that there was no “World War III” between the Sino-Western powers. Let’s keep emphasizing with our writers, as we know you have been, that this is a topic we’d like to avoid.