But that wouldn't be true at all. Being behind "terrain" doesn't tell us anything about the value of the cover save. The only thing that all terrain has in common, cover save-wise, is the 25% rule.
The only profession I know of whose job is to explain what ordinary sentences mean to a native speaker is that of lawyer, and we lawyers have for centuries decided that, if you are literally choosing between two interpretations that are equally probable except that one of them assumes the author put some words in that don't actually mean anything, you should choose the other one. It is only a rule of thumb, so it is not right 100% of the time (sometimes people do put in words just for the hell of it, obviously), but it's intuitively obvious to me that this is a good rule of thumb.
Of course, even if you subscribe to this rule of thumb, the two possible interpretations do have to be otherwise equally probable. So as to your point about the bolded text, I think that the bold stops in the right place even if I am right. I agree with you that the point of the bolding is to let the reader easily locate the main point of the rule, and in most cases all you need to know is that the target is behind an intervening unit. The cases in which the intervening unit is not tall enough to cover 25% of the target model, or that the shooter is high enough that the intervening unit doesn't cover 25% of the target model from the shooter's perspective, are rare.