On TV and articles I've read the phrase - 'the true outcome of a baseball play' is used to refer to a play that has one of 3 possible endings. They are a walk, a strike out, or a home run. We can all figure out what is different about these from other endings but does anyone know why they are referred to as true. Those 3 involve the fewest number of players but a pop-up to catcher involves the same, or a pop-up to pitcher. So all three involve the pitcher-catcher and the batter and no one else. But the question remains, why true?
This phrase appears to come originally from a Baseball Prospectus article in 2000 by Rany Jazayerli.
If you read the article, the language used is intentionally archaic/flowery for humor. It reads as a medieval declaration. As such, a phrase like "the one true sword" would fit right in.
He must be allowed to spread the Gospel of the Three True Outcomes to fans at major-league parks everywhere. He must be granted the privilege of rendering the opposing defense utterly irrelevant more than half the time. We will not rest until this is so.
We leave with a message for all loyal members of the RDFC: a great and glorious new era is upon us. Rejoice! A great man is here to lead us, and a new movement has been born.
Here "true" best fits as loyal, correct, or absolute. The other outcomes would be (humorously) regarded as pretenders or unworthy.
It seems the term "three true outcomes" goes back to humorous discussions on Usenet boards in the mid-1990's (see https://sabr.org/research/growth-three-true-outcomes-usenet-joke-baseball-flashpoint). The website baseball-reference.com attributes the first use of the term to Christina Kahrl in discussions about Rob Deer and his high propensity to have his plate appearances end with a walk, strike out, or home run.
In what I've read and heard via several sources through the years, the "true" has always been explained as indicating that the three results in discussion are the only true outcomes of the confrontation between the pitcher and hitter, unaffected by the defense behind the pitcher. (And yes, it ignores park factors like close vs. far fences, defenders catching balls that would otherwise go over the fence, and defensive misplays leading to inside-the-park home runs.)