The next line just simply states the obvious, if one of the two was used we would know who it was on because they would either be dead or babble, but from the statement it would be said which was used...I'm not sure it was. There was nothing about which vial was used.G you can't read just the red words. Read the next line also.
Besides I don't honestly know why this is that important, or for that matter what John would have to gain by faking a babble !
It is important because if it wasn't used...then John flat out faked it and has to be evil, there is no other reason for him to "pretend" to of gotten the babble.