I think that's a clever quip made by a man who just wanted to keep his marketing budget without being asked to justify what he spent it on.

Clever quotes are often repeated. That doesn't make them true for every context though.

When John Wanamaker said what he said I think he was talking about print ads for department stores in an era before ad metrics, large scale market research, etc. It was probably true then to be fair, but that doesn't make it useful now, or applicable to all advertising. For example, if I used this quote when I was talking about Internet adverts I'd just be wrong. It's trivial to track the effectiveness of an online ad

A lot of tech people have this rather Dilbert-esque view of marketing. In practice any halfway sophisticated marketing/advertising/paid search/etc. is a lot more quantitative and data-driven than they probably think.

I sort of admire marketers. They get so passionate about it! I can barely muster that sort of excitement for actually interesting topics.

For items/services purchased online. Offline, not so much.

Then take half of your advertising spend and redirect it into market research. (I.e. add on the bottom of your purchase receipts, email marketing blasts, etc, something like "$10 coupon if you tell us something about yourself"; where "something about yourself" = "which if any of these advertising campaigns did you actually see.")