Normally, I’d agree. Yet, Rumple is still the Dark One and therefore still immortal. I don’t think he died when he stabbed himself; I think he was simply transported to the underworld and would have been cursed to remain there for all eternity had he not be rescued.
I’m inclined to believe this. They can’t keep breaking their own rules: “oh dead is dead except in these following cases. Blue? Didn’t die just had her shadow ripped off so toss a coconut into a fire. No don’t ask us about Greg.”
I mentioned this theory quite a bit over hiatus but it could be a gender swapped (+son) version of Orpheus and Eurydice, in which Orpheus tries to resurrect his dead lover–which also fails because Orpheus looks back before fully out of the underworld, but in this case it could “fail” because the WW gets to Rumple first.