Definitely a script mistake, likely because the shooting draft was written/edited quickly. Specifically, Spock is in the landing party through the 5/23/67 Revised Final Draft and there is also no communications problem with the Enterprise. In fact, most of this script, from Acts 1.5 to 4, occurs on the planet, and Sulu and Uhura (on the bridge) have little to do other than to periodically talk to Kirk. When the 5/26/67 Second Revised Final Draft was printed (the draft they shot with), just three days later, Spock was moved to the Enterprise and Kirk couldn't now communicate with the ship, at least initially. This gave Sulu and Uhura things to do so their roles were expanded. Ultimately, this script had a few revisions through 5/31, which is the day they started filming.