Good question. I suppose it could hinge on if the cars are exclusively used in the ACO, thus they fall under the ACO's umbrella? AFAIK, the Gen 2 LMP3s were used beyond ACO series, but according to Wiki, the four Gen 3 LMP3 cars are all built to ACO regulations & to be used in ELMS & ALMS. Similarly, it states the current LMP2s are limited to a Le Mans spec in the WEC & the ELMS teams can switch between 2 aero kits.
Edit* The current LMP3s may be exclusive to ACO with wording describing the 4 current cars as, "the new
ACO Generation III LMP3 built car set eligible for the category".