It surely couldn't have anything to do with non-marketers overestimating their knowledge of marketing.
You wouldn't trust a marketer to give input on the job of computer programming. Why would you trust a programmer to give input on the job of marketing?
Something folks here are rare to ever admit. HN is populated with amazing people who see thru all facades and are completely incapable of ever being a 'victim' of advertising, because we all know better!
It definitely has an impact in general, that doesn't mean that every marketing campaign has a positive impact on their company. Though of course if you only ask the marketing department whether they are efficient, they are going to always say yes.
Probably because their marketing people are highly skilled at marketing themselves.