1) None of which makes what you said actually true. It's a perfectly valid approach to computer safety and is certainly a more thorough one, but it's rooted in an assumption that's just plain false.

That's not a criticism of it. In many ways, it's a more reasonable approach to take than the one I did. I mean, "Assume every gun is loaded." is still a valid safety tip, even though it's rooted in a false assumption.