Haven't read it, but one of the issues is that it just seems to be the same ****ty script that gets uploaded all the time. I could imagine Joker having different Persona, but having the other PT members seems a stretch. Crono would be believable if they just said the partner mechanic can be switched but the moveset would end up becoming unecessarily complicated. As for the Mii costume, they are just relaying info already known.
Joker is definitely not going to have more team members. It hasn't happened for Cloud, it hasn't happened for Shulk, it hasn't happened for any FE character outside of Robin's Chrombo, and by golly, it probably won't happen with Joker and/or Chrono either.
Hell the notion of him getting any persona that isn't purely Arsene is doubtful.
The idea I gleamed from the ****ty google translate is that Joker can use the Personas of all the other team members to alter his Neutral, Side and Down Specials, he does this in a similar way to how Inkling charges ink (Press B while Shielding, then make a selection from the wheel like Shulk's Monado arts).
Kirby can also do this (Neutral Special only, natch).
Of course, considering that Sakurai specifically pointed out that Kirby's a pain to develop for, would they
really let Joker have so many Persona, making more work to go into Kirby?
Also, Morgana and Jack Frost are (supposedly) Assist Trophies.