Sorry for the late reply.
Not unlockables, but characters that are new to Mario Tennis and have never been in the series before.
Rosalina, Toadette, and Sprixie are completely new to the franchise and never been playable. Dry Bowser and Bowser Jr. have been playable in previous installments.
If you look on the website, characters completely new to the series have a glowing "new" by them when you pass over them. Because of the typo, people theorized that there were two more unlockables.
He wasn't joking that he wasn't a fan of Fallout 4, but it was obvious that he was exaggerating and using caricature to poke fun at the game. Calling it a "failure" wasn't supposed to be taken literally.
You could argue that it's a jerk move to just make a jab at certain games or not (I don't care either way), but it seemed obvious to me that it was just a jab.