Well, it was easy for Spock. Which is consistent with him having done it several times before. Quite possibly, the know-how was never allowed to leak to the wider audience, and even within Starfleet very few people know how Spock accomplished the feats of "Tomorrow is Yesterday", "Assignment: Earth" and ST4:TVH.
We know very little about the motivations of the Probe. Spock recognized the voices he heard as those being made by humpback whales - but perhaps any whale would have done, and Spock just failed to realize that the Probe started talking blue whale a few minutes later, and switched to sperm whale not long thereafter?
We don't know if the whale sounds were actual communication, either. Perhaps they were just unintelligent noises, and the Probe was satisfied with hearing some being emitted from Earth's oceans, meaning the whole time travel effort was for nothing. Kirk's original idea of responding with "gibberish" would have solved the whole problem already!
Timo Saloniemi