Speaking for Dakka, the "rules-lawyering toolbags" are a small (albeit vocal and annoying) minority.
FWIW, I do think this question is a slightly complicated area and one of some ambiguity in the 6th ed 40k rules.
In 5th edition, monstrous creatures had a good deal in common with vehicles in terms of LOS & cover- to gain a cover save, an MC had to have 50% of its body (disregarding wings & tail, if any) physically hidden from the firer, as did a vehicle with the armor facing being targeted. Both of these stood in contrast to other models, which just had to have any tiny piece of the targeted model's body hidden, OR be in area terrain, OR be targeted between raised elements of a piece of area terrain ACROSS that terrain, OR be targeted through/between models in an intervening unit.
Now, in 6th, the rules have been tweaked a bit and the way different models get cover is more similar to one another. Now MCs can get cover saves from area terrain the same way other models do, and vehicles are the only exception. Now every model is subject to needing to be 25% hidden (25% of the targeted facing still, for vehicles) if they're not in area terrain, as opposed to any tiny piece of the model's body (or 50% body/facing for MCs and vehicles), like in 5th.
Three other changes:
1. Cover saves are now model by model instead of "all or none"; in 5th, if half or more of the models in the targeted unit were in cover from the perspective of half or more of the firing models, the whole unit got a cover save. Now each model either is in cover or not in cover.
2. The other thing that changed in the requirement above is that instead of "half or more of the firing models", as in 5th, it's "at least one" firing model.
3. Area terrain no longer generally confers a cover save for shooting ACROSS it. If the targeted model isn't actually in it, it needs to be 25% hidden. With the exception of Forests (see page 102); which have a specific rule that if at least one of the firing models is drawing LOS across a forest, cover is granted.
Other than area terrain and shooting across forests, the only other time you can get a cover save in 6th without being 25% hidden, is when you're being shot across/through an intervening unit. Check out page 18 again. While it does say that a 5+ cover save is conferred in the same way as behind terrain, the next two sentences tell us that "Similarly, if a model fires through the gaps between models in an intervening unit, the target is in cover, even if it is completely visible to the firer. Note that this does not apply if the shots go over the unit rather than through it."
So, we know that a model which is totally visible to the firing model can still be in cover if LOS is traced to them between to models in an intervening unit.
The area of ambiguity comes in determining WHEN LOS counts as being drawn between the models, as opposed to OVER them.
Back in 5th the guideline that was generally used was to draw LOS from the firer's eyes (which is still the standard, see p8), to all parts of the target's body- head to toe. If you could draw a line from the eyes to the bottom of the feet without that line going through the plane drawn between the tops of two models in the intervening unit, you had a clean LOS and no cover.
Now in 6th, it would appear that the same procedure should be used, but instead of the bottom of the feet, we've got to go with whatever point on the model constitutes the 25% line for that model's mass/silhouette, as the requirement is now 25% hidden. And it looks to me like that empty air between models in an intervening unit is to be treated (per page 18) as something that confers cover saves.
At least that's my take on it. I'd be happy to hear other thoughts.
