It...is, though? Somebody took the US version of the ROM and modified it, to replace all the English-language audio with the original Japanese. (From what I've read, it also apparently corrects several errors in the display and subtitles.) I guess it might not be the first thing someone would think of when they hear "ROM hack" -- it's not, like, Mega Man in the Mushroom Kingdom
or anything -- but it's still an unofficial modification of a ROM. Like, say, that MSU-1 version of Mega Man X3
that replaces the SNES soundtrack with the terrible PS1/Saturn version.
Including multilingual audio on games is still a pretty recent development, and largely a result of platforms (PC and newer consoles) where games run from a hard drive and therefore aren't constrained by the storage space limitations of cartridge or optical media. Persona 4 is about 3.2GB, give or take; I'm not sure how much of that is voice audio, but I think there's a significant chance that multilingual audio on the PS2 would have meant using a dual-layer DVD instead of single-layer, and simply would have been too big for a Vita game, full-stop (a quick glance at Vita game sizes
indicates that being bigger than 3GB is atypical, and I don't see anything in the list that's bigger than 4GB).
Persona 5 doesn't need a modified version to restore the Japanese audio; it's included as an option in the base game. That's pretty common now, especially on something as super-Japanese as SMT, but the gap between Persona 4 and Persona 5 is eight years and two console generations. (There's a PS3 version of Persona 5, but AFAICT it was never localized outside Japan.)