@Charles - The _absurdity_ of the original post is evident: "There is absolutely no way to ensure there are not backdoors. Same goes for Mac OsX."
That is the statement of someone wearing a tinfoil hat. Such idiots are not satisfied by rational, because the 100% unequivocal proof lies with the defense. The accuser merely has to judge the effort as futile or erroneous, and arbitrarily widen the burden of proof as necessary. It is an unprovable assertion, by design (perhaps more subconscious malice than overall stupidity).
The negative statement, as per example, is analogous with this: "There is absolutely no way to ensure there are not backdoor probing aliens."
I do not dispute the veracity of the original premise, in Windows' case by stupidity (not malice)... the clumsy assertion warrants anyone's criticism.
I will not get into the vaguery of the initial statement, regarding backdoors, be they system-provided or by third-party tool.
The absence of a guarantee does not give you the logical weight to make ridiculous claims. This is why we have idiotic arguments with Global Warming (I grew up last century, translate as you wish).
I fail to see application of Reductio ad absurdum. Quite simply, the original poster used idiotic phrasing.
The implied stupidity of the original poster is not finite by any means; furthermore, I'd say that he will always try to outweigh any evidence contrary to his original statement – because it would upset the view of government spooks as being lucky instead of capable... to bring it full circle with the actual article.