Well because the truth is revealed in episode 15, as opposed to episode 6 when she first appears, it'd still be considered a spoiler. In terms of the light novels, she first appears in volume 1 story 3 (corresponds to episodes 6-8), but the events of the most recent episode (15) correspond with volume 3 story 1 (likely episodes 15-16). I don't recall if she is mentioned at least in name or description before volume 1 story 3, but she at least isn't present until then.
Heavy Object's volumes are typically written as 3 stories and an epilogue, with a prologue to kind of describes the overall theme for the volume.
I think it is not necessary. Because the truth was revealed recently in episode 15 of the anime
Well, not everyone has watched up to episode 15 yet. A similar line of argument would mean all spoiler tags are completely unnecessary, since spoilers (barring internal leaks) are by definition always revealed in the latest episode/movie/chapter/volume/etc.
"Nope, no need to spoiler tag Mami Mogu Mogu scenes, since it's revealed in episode 3 anyway." Not.
Of course, most other spoilers like the deaths of significant characters tend to be more dramatic (and knowing them before they come ruins the dramatic tension), so people tend to appreciate if those elements are spoiler-tagged. Breasts size... okay, it ruins the joke a bit but I guess it does technically qualifies as a spoiler in the same vein as trap or reverse trap side characters.
