Backtracking a bit to say that I completely agree with you here. The volume takes it's title from the very next chapter after Ace has been mortally wounded. Chapter titles (or titles in general) to me are certainly part of the artistic product - the artist chooses the title just how he chooses to depict everything else in his story. Aas you said the chapter (and thus, the volume title) is not a spoiler, but a poignant confirmation of what the audience has feared after finishing the chapter before.
I think the question if an author/storyteller can spoil his own story is an interesting one. I'd say he or she definitely can, if he isn't responsible with regard to how his audience conceives his story. If Oda vaguely hints at things he may do at some point in the future (f.e., stating that a crewmember will die when referring to the Going Merry) or if he enhances his storyworld by giving minor information or confirmation about characters or events outside of his actual story, it's not a spoiler to me. But let's say, if Oda would have irresponsibly stated in an interview shortly before Ace's death in the manga that he would kill his character off, that would definitely have spoiled the dramatic twist for any fans reading that interview. Obviously, Oda would never do such a thing, I'm just saying that it's not completely impossible.
I know it's a matter of semantics, but I don't think an author can spoil the story from within the story. By definition that's just not what it is. You can argue that it was the wrong decision to give the game away so soon (Stephen King, for example, has a habit of casually dropping sentences like "before the question would occur again, death would come between them" in his narration in a way that I usually feel the story would be better without) but by definition it's still not a spoiler, just a bad decision for tension or anticipation building.
Interviews and so on are definitely a whole different board game. It's outside the story and in a totally different context, so the author might feel the need to generate marketing hype, or go to far in explaining things because they assume the broader interview audience is less invested and needs more explanation, or just straight up make a mistake because interview answers are inherently less revised and edited than a final, published manuscript. But then, it's so hard to judge in that kind of circumstance what even is intentional or accidental and how much of what's being said an author intended the hardcore fans to read into and give the game away for themselves. It all gets so complicated when you have to guess at that kind of thing.