[djrobstep]

I'm not sure how that connects at all - "the purpose of a system is what it does" (https://en.wikipedia.org/wiki/The_purpose_of_a_system_is_wha...) isn't making any claims about correctness.

[perl4ever]

I would say that the purpose of something is what is proper or correct for it to do.

I think the slogan should not be taken literally and means more like "don't let the (possibly many-faceted and disputed) purpose of a system blind you to what it does".