Three things demonstrate their belief that it was a mistake.
Firstly, the response in 2013 from then lead developer Greg 'Ghostcrawler' Street when asked how he felt about Pandaren neutrality.
To quote it here, "How do you feel having Pandarian shared between both factions has worked out? Would you do the same if new races were added?"
Originally Posted by Blizzard Entertainment
Not a fan of it overall. We just didn't want to deny anyone Pandaren.
Secondly, Pandaren were implemented in 2012 and were announced in 2011. It is nearly a decade since they did the experiment with neutral races. Since Mists of Pandaria we have had four expansion announcements, only one of which introduced new races but did introduce far more new races than had ever been added at one time before. None of those races were neutral despite some, such as Nightborne, clearly being able to go that way.
And thirdly the rationale provided by current game director Ion Hazzikostas on the two occasions he was directly asked why Void Elves and why not high elves? Because giving a duplicate of an existing core Horde race to the Alliance undermines the distinctiveness of the factions.
Pandaren are a race available to both sides. A race that is identical on both sides erodes the distinctiveness of those two factions, particularly as those two factions's distinctiveness is predicated on their racial composition and Blizzard recognises this with their refusal to make High Elves cross faction. If that is the rationale for denying high elves, then it was the impact of neutral Pandaren. The damage done though was limited because this was with a race that was brand new to the game and whom neither faction had a claim on and whose entire story was based on balance and neutrality. In contrast Blood Elves have been Horde for over twelve years now and it would be far more damaging to faction diversity should they be made available to the Alliance.
Pandaren neutrality is not a precedent. Pandaren neutrality was an error, an experiment that didn't work out. You don't point at mistakes and say that proves it can be done. You learn from mistakes and you don't repeat them.
Void Elves might be judged to be the outcome of this philosophy.