This is only true if you are an omnipotent master of the universe. It is because only someone who has mastered all the knowledge there is, can make statements like this with such absolute certanity. So you are a god, in a way.
It's been almost 300 years since epistemology was invented as a discipline, and yet people can still say things like this and expect to be taken seriously."mind-virus" is an understatement.
This has nothing to do with the topic.
This is simple logic. IF there is an extremely advanced being who has the power to affect time-space continuum in every imaginable and possible (or impossible) way, then there would be no way of detecting it, unless it wanted to be detected by design. Complete control, complete power.
Whether it wants or not to be detected is a topic for another discussion.
Apart from the fact that very few people actually use brainwallets (I'm being very liberal with my interpretation - one's brain and forehead are really two completely different body parts), you can transact with bitcoins right now without ever seeing a private key. And you always will be able to. In fact, it is likely that in the future, most people will never see a private key, instead trusting their bank to handle all the "technical" stuff for them (which, considering most of the general public has yet to figure out how to download porn without infecting their computers with half a dozen viruses simultaneously, is probably a good idea). 
