This confirm covers the literal seed — that the Sea-Forest stone is Joy Boy’s apology poneglyph.
Why this matters
A chekhov-explicit seed fused with dialogue-from-a-dead-character: Oda lets a Void-Century figure speak directly through an indestructible stone, and Robin’s on-page question (“who is it apologizing to?”) is the reader’s own question planted as bait. The name “Joyboy” is installed here, ~400+ chapters before the Gear-5 / Sun-God-Nika reveal canonizes who he was. Cross-link Poneglyph, A Poneglyph On Fish-Man Island Holds A Key To The Void Century.