> > But testing ensures no such thing

> Yes it does. Empirically.

Your claim was "Testing ensures that the knowledge is checked." And, yes, testing ensures that the knowledge is checked - by the test.

mrbrowning's claim is that "Static typing ensures that some sliver of that knowledge is explicit and checked." That's also true (despite your reply of "No"). And note mrbrowning's use of the word "explicit". Type knowledge is more explicit than a test. It ensures that this data structure is an instance of the specified type. A test proves that in this set of circumstances this data structure behaves the way we expect, which is a weaker proof.