You're arguing that the sentence contains enough information for the author's intended interpretation to be understood (which is true); in practice, however, customers expect developers who answer questions at small companies to be more knowledgeable than they are, and often take comments, advice, or instructions almost literally. Along similar lines, most customers of most software companies are not developers, and therefore are not used to reading text carefully, like we are.

Regarding the author thinking his customer was pissed, this helps prove my point that the author needs to work on his written communication skills. He not only fails to realize that he really did phrase his words incorrectly (instead of the customer misunderstanding the sentence), but also incorrectly misread the customer's reply as being "pissed."