It could also very well be the fighting game for P5, it's more than time for that. 

Something like Persona 5 Suplex, but Suplex was already in the name of the previous one...

I don't believe in a game called "Persona 5 Switch", they like to find weird "stylish" names, so it's unlikely. And I would find weird to have a different name if it's the same game as Persona 5 Royal, they would rather just announce the Switch version during the complete reveal of this one.

Oh sweet another Persona 5 game for PS4 :P

Maybe some exclusive 1st party content?  Nothing major, but Pinky is right here.  There is no way they would release the base version on Switch when the Royal is coming to PS4.  It would be counter productive to say the least.  I have a feeling it will be an all or nothing type of port if it comes to Switch...and that is still an IF.

Page's source code hides the following sentences:

【予告状】

2019年4月25日。

世界の認知を覆す、新たな怪盗団をお見せしよう。

我々は会心を求む声に躍動する。

今一度、両国国技館にて諸君の心を頂戴する。

心の怪盗団より

Which Google translates as:

[Notice letter]

April 25, 2019.

I will show you a new monster thief that will overturn the world's cognition.

We are excited about the voice of the meeting.

Once again, we will receive your heart at the Gokugikan.

From the Phantom Thief

The Ryogoku Kokugikan in the message is a big sumo arena in Tokyo. Also hosts some other wrestling or boxing competition. 

Feels like a fighting game...

And by the way, the previous Persona 4 fighting games also released on Xbox. So it could easily release on Switch, and do the link with Super Smash Bros.

EDIT: Persona 5 Sumo? 

Woo, I'm all for another Persona fighting game.

P4U was a ton of fun.

If it's a fighting game....THAT game will be multiplat most definitely. The Persona fighting games are awesome.

Very high chance

This

So in the end, it still use more letters than just Persona 5?