Probably because there's a scene identical to this situation in the manga, and it's kind of big deal plot wise, but it's not some kind of big reveal.
Personally, though, this single image hardly means anything significant unless you already know the scene in question. I don't think "it happens in the story" automatically makes it grounds for the spoiler tag, especially since to someone who hasn't read the manga it doesn't even look like anything important.
Probably because there's a scene identical to this situation in the manga, and it's kind of big deal plot wise, but it's not some kind of big reveal.
Personally, though, this single image hardly means anything significant unless you already know the scene in question. I don't think "it happens in the story" automatically makes it grounds for the spoiler tag, especially since to someone who hasn't read the manga it doesn't even look like anything important.
I remember a similar argument being made for Magica Madoka "Spoilers" for Homura having glasses. In the end, it was ruled that any spoiler should be considered spoiler and on grounds for spoiler tag.
The counter-argument was that "oh, well if you don't watch the show or are behind, you would think it was fanart of Homura with glasses" but it was a pretty big plot reveal so nontheless, spoiler.
Honestly, this is the sort of situation where I think throwing the spoiler tag on it can be questioned. If Goblin Slayer was next to her I would say it was a spoiler, but all I see is Priestess sleeping in the buff. This could easily have- and may well have- been drawn by someone who just wanted to draw a sleepy Priestess naked in bed with no knowledge of aforementioned spoiler.
I agree that only if Goblin Slayer were in the picture would I agree it was a spoiler, more so if it also revealed why they were sleeping together. This is just a nude Priestess without additional context.
If we're deeming this a spoiler, then there's a couple of images of High Elf Archer being assaulted by goblins that should also get the tag, as that too is something that happens in the story.
