The fizzing vinegar just usually means there are carbonate based things in the rock and that it will likely raise the hardness and pH of the tank water. Usually this is not something you want, thus the general idea that the vinegar tells you not to use a rock. But sometimes people -want- to raise their pH with something and these type rocks are sometimes the way they do it. Just didn't want anyone to think its worse if they read the word "safe."
Other thing to mention here is that directly boiling rocks is considered risky. There's a small chance (and in fact your bubbles show you got a bit closer than most!) that air, liquid or gas trapped inside a rock could expand and explode the rock if its boiled. Its better to just pour boiling or just hot water over the rocks but not submerge them, so they can cool right back down.
~~waterdrop~~
