I think this blog is missing the point.. While designers like to pretend users are stupid (as interfaces need to be built for least computer saavy) - users in general are not stupid, and can make the association behind logos on a web site. Its clear they're either clients, or its press, depending on the context. But equally so - its not relevant, as they do in fact (as you mention) lend credibility / create trust, which will increase conversion rates.
True. Especially for sites like Heroku. Their target audience obviously knows what this is all about, right?
I agree that users aren't stupid, and most people can tell what it's supposed to mean. That doesn't make it any less lazy or dishonest. Some of the examples on the blog aren't that bad (eightshapes.com in particular) and provide appropriate context with explicit labeling.
I find it especially egregious when there's a group of logos purported to be about press coverage that don't link to the coverage.