If there's a benefit to the liquid, I have no clue what it is.
If it's bacterial there's a good chance it is dead at the time of use, if it contains nutrients for plants it will be gone with water changes.
Assuming it hasn't leaked out of the bag in the first place.
Any benefit I get from it, it's from the actual substrate itself and the high CEC.
I want to say I recall it being tested, but I cannot remember where, and do not have time to look it up, so take that with a grain of salt.
I have used it. And am currently using it (until I break the tank down that it is currently in). But I have so much new aquasoil stored in the basement I'm unlikely to use it.
If the liquid was nutritious, plant wise, it would end up in the gravel (CEC related) but I think I remember seeing a breakdown of it, and pretty sure it wasn't beneficial to the plants outside of maybe a few micro nutrients.