Its vinegar, just really strong.
But vinegar, or any acidic liquid for that matter, only has its own limited uses. It's good for cleaning water residue, it's even better for cleaning oxidation off metals, but it's not good for washing the burnt smell out of foam. It would've been understandable to suggest some other "grandma's recipe", like dumping 5 kilos of soda on it and leaving in the closet for a month, cause it works with my stinky sneakers, but c'mon man... concentrated vinegar and foam...
Yeah I don't know about any of that stuff, I just know its not foam, and its heavy and works really well.
It's just a different type of padding. Same polymer, but instead of foam it's packed in panels as fibers. Kinda feels like very shitty wool. This stuff is even cheaper than foam.