(1) It specs a far more accurate movement than COSC certification. 0/+5 vs. -4/+6. Maybe that doesn't matter to you personally, but it's a 50% better accuracy range (5 seconds against 10), so that's not marketing. It's an increase in accuracy, much like Rolex's superlative chronometer rating of -2/+2.
(2) People experience magnetic fields everyday. Sitting at my desk right now I count 8 magnets within two feet of my watch. Not that it's much, but human beings also emit a magnetic field. 15,000 gauss may be over the top, but it's nice to know that if I leave my watch next to my laptop that it won't become magnetized. Or, if, you know, I am at a hospital where there are machines that emit a large amount of magnetism, I can worry about the task at hand.
So it's not "pure" marketing. Their new deep dive watch, yeah, that's pure marketing. But while 15,000 gauss may be more than necessary for most people (though, if you've ever had an MRI, you've been subjected to around 15,000 gauss), it's a useful shield. And if you don't value mechanical accuracy, well, I don't know what to tell you, but lots of people do.
Sorry for the aside, but sometimes this faux-iconoclast cynicism needs to be called out. Of course there's a marketing element, but it's not purely for marketing purposes. Both the accuracy standards and magnetic resistance are accomplishments in watchmaking that are useful to watch wearers.