On September 5th, 2018, a Nintendo Direct was announced for the following day.
On the morning of September 6th, 2018, this Direct was postponed due to an earthquake that hit Hokkaido, Japan. Which was a stupid thing to do since video games are more important.
Much of the Smash community speculated that the delay absolutely had to secretly be Smash-related, confirming either Isaac (because he's an Venus/Earth Adept) or Skull Kid (because of the Moon destroying Termina).
Many argued the opposite, that the natural disaster on it's own isn't reason enough to postpone a 30-minute long video game presentation. Or if sensitive material truly was a cause in the delay, it HAD to be Smash Bros, and couldn't have been the other upcoming game centered around an entire landmass being destroyed.
The direct instead aired on September 13th.
Isabelle was in as a playable instead, confirming that the people who thought it was delayed due to an Isaac/Skull Kid trailer were incorrect. Also,many felt that Isabelle was underwhelming (though not necessarily bad) compared to previous reveals like King K. Rool and Ridley, though this is likely a combination not just made by Isabelle but by the delay causing hype/speculation to spike, the direct consisting of mostly ports with the new Animal Crossing, Yoshi's Crafted World, and Luigi's Mansion 3 being the only new games, and possibly the largest that the online system was underwhelming and generally a disappointment.
And then Sakurai confirmed the Direct in fact WAS edited because of the earthquake. But it doesn't matter because Isaac was disconfirmed on November 1st's Direct, definitively proving that he was never intended to be revealed in September.