Not to mention the trend that more and more dollars are being spent online, not offline.
I don't know why you think the assertion is absurd. It seems like a pretty sensible assertion to me, but I don't know very much about marketing. Do you have time to share some of your marketing experience that makes it so obvious to you that online and offline ads aren't in competition?
It seems like, at a minimum, online and offline advertisers are in competition. If online ads are sufficiently good at getting customers to buy from you, and your competitors are only advertising offline, then you're going to take market share away from them — and have more money to plow into online ads. If particular advertisers can shift part of their advertising spend between online and offline, that represents a more direct kind of competition between online and offline advertising.
I think the stuff you wrote is mostly actually in the article.