I think that used to be the case with BL but nowadays with the success of Horus Heresy they found a singular narrative much more lucrative. Look around a lot of 40k people online only care for the lore.
The "singular narrative" has always been a goal. The advertisement needs to reflect the product, after all. And they've always strived for this, but ultimately, when discrepancies occur the official line from GW has always been that rule book lore trumps anything else in terms of canonicity.
While the peripheral materials should work to reflect the lore of the rule books, my point is that ultimately anything that gets introduced in a novel that doesn't jive with the broader fluff is irrelevant since it can just be retconned away later or outright ignored.
In terms of details that actually matter, not much has really changed from the lore established in yon olden days of Rogue Trader, and often things that have been forgotten from that era are revived and brought back. The only thing that has really changed permanently has been the title of the Imperial Army and the rank titles of Space Marine chapters. And the color of the Dark Angels. Besides that, there have been two new armies and that's it (Necron and Tau. Even the 'nids were present in Rogue Trader)
Besides this, the rest of it is all there. Gene Seed, the Heresy, the battle on board the Vengeful Spirit, multiple factions of Eldar, the Ruinous Powers, Ollanius Pious, the Primarchs, Thunder Warriors, even a bunch of the named Marines, all of it has been there since the beginning.
Any dumb stuff in a novel really doesn't matter unless it gains so much traction in the fandom that it ends up getting a model and makes it's way to the tabletop. Hell, even the dumb shit has been around since the beginning. In an issue of "Challenger" magazine by Games Design Workshop there was an article about an all-female chapter of Space Marines, so people have been whining about that since 1989. And if in over 30 years THAT hasn't bled over onto the table, none of the other dumb shit will either.
All in all, at the end of the day GW doesn't care about any of the periphery. All of those products are just there to sell
toys autism soldiers. And to make that work, the advertisements need to be as close as possible to the real lore so that when people cross over and finally buy a rule book they aren't confused.
Edit: Even the concept of Sisters of Battle goes all the way back to Rogue Trader. In the very first book, too. Hit up the trove and check the PDF of the first edition. One of the final pieces of art depicts Sister Syn shooting a member of the Rainbow Warriors in the chest. The nuns just didn't get a proper army list because GW took too long hammering out the details that it got pushed back until after 2nd edition was brought to market.
