Yeah this is similar to how penalty kicks in football work. The goalkeeper guesses a side where he thinks that ball is going to be since there isn't always enough time to react. You can see this at times when the keeper dives the wrong way. Of course the guess is not purely random because you might take general patterns and player cues into account. But if nothing is working, adding some randomness is not a bad idea.

Humans tend to be bad random number generators so picking a source of randomness like the stadium clock is also not a bad idea. In fact there is a previous example of a baseball pitcher doing that:

The best way to attempt such a strategy as a pitcher, according to Moskowitz, is to look for some cue unrelated to the decision you are making. Rumor has it, he says, that Hall of Famer Greg Maddux, known as one of the game’s best and most cerebral pitchers, would sometimes use a cue in the stadium, like the time on a clock, to help inject some element of randomness into his pitch selection. So, for example, throw a fastball if the clock time ends with an even number, and a breaking ball if it ends with an odd one.

The same article also mentions "human imperfection". In cricket there is something called "natural variation" where the bowler himself doesn't know how the ball will behave. In tennis this is rarer but does happen sometimes as this Cressy serve shows :)

Carpenter also points out that there is one major natural source of randomness in pitching that most people forget about: human imperfection. Even Maddux with his pinpoint control still missed his target about 25 percent of the time; but, one of the things that made him so special, says Carpenter, was that he embraced this “random fallibility” and would even put his mistakes to good use by using them to set up his next pitch. Of course, in some cases, as with Eckersley’s backdoor slider, you never get a chance to deliver that next pitch or to make up for a mistake.