Rule 5.09(c)(4). Hands up anyone who had ever heard of this rule before Wednesday? If you put your hand up, you’re almost certainly lying.
In short, for the Washington Nationals, based on how the umpires called the play initially, they applied the rule correctly — the third base umpire called the runner who advanced from second to third base without tagging up out at third base as the third out, but not before the runner who ran home from third base without tagging up scored, so the run counted.
The run counted because the third base umpire didn’t believe that Ehire Adrianza made an obvious attempt to appeal that the runner who left third base early for a “fourth out” which would’ve negated the run.