Do you intend to provide some support for your idea that "it's useful to know about the performance of idiomatic software"?
No, they're dismissing it as useful for answering the questions we tend to care about when evaluating the performance of various programming languages. One such question is "If I choose a given language, how far will I be able to go with just idiomatic code before I need to invest in expensive optimizations (most of the expense here is that we're spending time optimizing instead of iterating on features)?". "Real code" is a function of this concern, so measuring it doesn't help us answer this question (i.e., if a language performs poorly on this question, "real code" will be more optimized and less idiomatic). Further, microbenchmarks don't measure "real code" either they maximally optimized code, which is what the OP was complaining about.