I agree that this particular agency was way out of line with their specific comments, especially likening the 2013 situation (one of real achievement, leadership and abundance) to 1997, the nadir of Apple innovation and status. But you are way out of line in pontificating that ad agencies or marketing in general should have no say in product development, how a company is run, or future decisions. This shows a fundamental lack of understanding about what real marketing is, and is 180 degrees from the truth about how great business/brands run. Companies that flatly ignore feedback from their sales and marketing departments go out of business almost as fast as companies who are slaves to them.
Steve Jobs himself was a master of marketing, and used it as effectively internally as he did externally. A classic example is the "Think Differently" campaign. It undoubtedly did a great job of beating the drum for Apple when they had very little product innovation to crow about, but more importantly it internally defined for Apple and it's employees who they were, what their purpose was, how singular that is, and how high the standard was for their collective and individual conduct/productivity moving forward. Branding isn't just putting lipstick on a pig. It's deciding who and what you want to be, then manifesting it and letting the world know about it.
Jobs often demonized mindless devotion to focus groups, design by committee, and slavishly following trends, and these have often been wrongly characterized as "marketing". But that's as empty headed as saying Android is open and Apple is closed. A bunch of unproductive yammering. As a discipline that is focused on understanding people (consumers), what they want/need/think, and the current state of the market/competition in terms of satisfying/influencing those things, marketing is uniquely qualified to positively influence everything from product development to employee behavior, as well as external communication and sales.
Highly technical people often aren't intuitively gifted at knowing/understanding people, and "people" people often lack technical savvy or practical imagination. Apple was blessed with a leader who had both amazing facility for technical subjects as well as deep, intuitive understanding of what people want/need/think. And that blessing has led to the curse of Apple fans, and perhaps Apple itself, believing that ALL product engineers should be able to perform such a freakish double whammy. But Steve Jobs' don't come along that often. And even geniuses like Jony Ive need counter-influences like Jobs to keep them balanced or, left to their own devices, they can create things like the chilly, under-intuitive, emotionally "flat" iOS7.
When practiced by talented, disciplined people, marketing and engineering are not natural enemies. And good companies know that, which is why most of them leverage the strengths of a wide variety of "thinking types" to innovate and execute both products and communication. Balance is key to success. And since uber-hybrid all-in-ones like Jobs are few and far between, being open to the influence of a talented creative/human/marketing voice is a healthy practice. Clearly in this case, the specific TBWA partner who penned that letter wasn't that voice (though to be fair, many past TBWA/Chiat/Day partners have been). Hopefully Angela Errands, and Apple's future agency partners, will be.