It's not entirely new, no, but it's new like the iPhone was new. None of the component ideas in the iPhone were 100% new, but it wasn't possible to build one at a consumer price-point until certain preconditions, both technological and economic, were met.
I'd claim this: marketing more than most disciplines lives or dies by the state of communication technology. The best marketers in any generation know how to use that technology to their advantage.
But communication technology is radically different today than it was even 20 years ago, let alone 120 years ago when PT Barnum was alive. For one, it's more digital and more technological. It's also changing at an accelerating pace. 10 years ago neither Facebook nor YouTube existed. 6 years ago Twitter didn't. Tumblr, 5. Pinterest, 2. Instagram, 1.
Or, think of it this way (as a marketer might): the rate at which new marketing channels are being created is accelerating. We're talking channels that can potentially reach 1MM, 10MM, 100MM people sprouting afresh every year. That's not the world PT Barnum lived in.
Why is is so crazy to think that this new environment results in a new breed of marketer, more at home in bits and bytes than in creating hoaxes which spread through poorly fact-checked newspapers and traditional marketing "stunts?" And that therefore engineers -- the same people who are creating these new channels -- are perhaps best adapted to this new environment?
It's also presumptuous to think that growth hackers -- and I'd include myself in that lot -- aren't familiar with PT Barnum, David Ogilvy, Jack Trout, or other famous marketers.