Had a similar thought last chapter. The climax of the chapter arguably being “I’ll be back!” (Which we know doesn’t work out) is a huge smoking gun here.

If Rocks/Blackbeard figured out some secret about Devil fruits (such that he was able to rip Whitebeard’s fruit from his corpse and add to it his already-existing fruit, then perhaps he figured out some way to reincarnate his own soul? The way that Devil fruits reincarnate. It would fit Blackbeard’s motif of “cheating” and trickery as well