We read the news to (try) to get an unbiased view of events (the baseball game, the hurricane, the battle, ...).
Bias exists, so we read multiple view points to get perspetive from many positions; and weigh their thoughts and ideas to form our own consensus (influenced by our bias, naturally) on the issue.
If the WaPo's editorial board endorsed Trump, it would be a significant event, quite inconsistent with their reporting, and their editorials, which would cause many to read the body of the endorsement to see the basis for the change; if compelling, and cause some to re-think the election.
WaPo endorsing Harris would be consistent with the body of their reporting and their editorials.
Announcing "No Endorsement Policy" this close to the election would be like MLB announcing "No World Series Winner in 2024" with the Dodgers up 3 to 0 in the series, right after Hal Steinbrenner met with the commissioner...