(1) The aftermath and the Big Reveal are two different things. You can easily mention a cult rebellion based around the Cult of Saint Drusus without giving a big reveal for the driving plot of the trilogy, the BBEG for the trilogy, or the deeper levels of the conspiracy that frame those adventures. And for the record, we're not talking about spoiling a 20-page module printed in a setting book, we're talking about spoiling the major point of an adventure trilogy, each part of which had to be purchased separately. Big difference.
(2) The setting doesn't stand on its own if the setting is unified, and it is. OW printed descriptions of the major setting areas for DH, RT, DW, BC, and itself, which is honestly something completely new in the way FFG is presenting settings - it's not like DW goes into setting detail for Calixis or Koronus,a after all. And if the Spinward Front needs to stand on its own, sure that's fine - it's closely tied to Calixis, that doesn't mean the DH-type machinations of Calixis need to be offhandedly mentioned in discussing a potential setting for Imperial Guardsmen. One shouldn't assume that people who buy one FFG line are reading others, but that would sort be be the hope, wouldn't it? Especially on profititability and brand goodwill levels?
(3) There doesn't need to be a statute of limitations on major spoilers, that's idiotic. So because DW's TEP has been out forever, you think it's OK to spoil the major points of that adventure in a future publication? That's bull. I would think FFG would want people to keep buying old adventures and running them, but if the plot points are spoiled in general sections of future products, what's the point?
I don't know the end result of the Haarlock trilogy, I'm currently playing in it, it's been out for much longer than Apostasy Gambit, and I'd still be pissed if it was throwaway-spoiled in a new product I paid money for, as if the spoiler didn't matter one bit.
And in the end, that big reveal added nothing to OW, it was just a crap editing decision. See point #1.
To my knowledge they haven't spoiler warned anything before. Also to my knowledge, FFG has never published such blatant spoilers in another game line before, either.