The thing with saying, “Why didn’t they announced Erdrick after spending 3 minutes on DQXI?!” It makes perfect sense and isn’t questionable.
Not only does Erdrick not show up in DQ11, that was a spot meant for a game from a 3rd party, and it’s a 3rd party who hasn’t always been on Nintendo consistently. While it is their IP, you don’t steal that thunder to say, “Hey, forget about that. Here’s Smash!”
While it’s not exactly the same thing, it would be like if Marth wasn’t in Smash, and when they did a presentation on Three Houses they were like, “Oh yeah, one more thing...” and then revealed Marth. It wouldn’t really make sense. It isn’t like just because Erdrick is also a Dragon Quest protagonist it make sense to follow up DQXI with him.
Frankly, I think the fact that they hyped it up as a “legendary RPG series” cemented my trust in the leak.
Not to mention, they got Smash out of the way by saying, “Stay tuned”. They weren’t going to reveal him after saying that.