Allow me to rephrase: Brewer's yeast CAN come from the vats or from other sources; nutritional yeast, the stuff sold in Health Food Stores for human consumption is more specific in type and source. Not necessarily the same thing, at all. And I don't really trust pet food companies, which are far less regulated, to go for the more expensive option.
So let's just say you and I disagree on the subject and let it go, OK? You've got your mind made up already; I don't see any point in continuing to try and discuss the subject.