Ultimately, marketing a novel has to be a tricky thing - double so when it is set in a "shared universe" like Warhammer 40k. As such, I agree with the main thrust of your argument. Unless the novel in question was about how the Legion of the Damned went from "normal Chapter" to "deathless revenants", I would not expect an entire novel to be about then, no. It wasn't the title that steered me away, though. I was an art major before I became a career military guy, and I think the cover - more than anything else - led me to buy into the complaints I read re: the book being advertised about one thing, but being about another. Don't get me wrong, the cover itself looked amazing, but the absence of Kersh and the World Eaters probably reinforced that unfortunate meme in the back of my cranium.
Ultimately, though, we're just shooting the shit. I don't pretend that you have full control over the marketing and decoration of your novels, and I'm certainly not trying to assign blame.
the sample chapter alone they put on the website was just right. I bought the novel on its strength alone. I don't know how other readers felt, but I thought it was just right for setting the tone of the novel and letting you know who your protagonist was going to be. I mention this because I bought this novel two weeks ago - again, on the strength of the sample - and, looking at the website now... the sample is gone. And the two pages under the Space Marine Battles shortcut don't include the novel.
I'd shoot a friendly message off to the website folks, Rob. I can't imagine I'm the only person who happily found out about this novel entirely the wrong way!