completely unsubstantiated gut feeling type claim... I'd be rather hesitant to put that in a typical soft-water tropical tank.
Aside from that, I've no clue what that rock is. I think the vinegar test is a bit over-rated, as it tends to have a _lot_ of false-negatives (rocks that have high carbonate content, yet don't appear to fizz immediately...), and is also prone to misinterpretation.
If you have easy access to a strong(er) acid, I highly recommend you use that.
Otherwise, I'm partial to the scratch test, I think it's easier, both in interpretation and execution. Take your rock, and see if you can scratch it with the point of a knife, a nail, etc. (pretty much anything made of steel....)
If the rock scratches, it might not be safe (further testing necessary), if the rock does not scratch, it's almost certainly safe.