I'm pretty sure slim stalky was turned into the Jack animatronicI'm not sure, but it reminds me back in 2017 when Slim Stalky(Gemmy's answer to Slender Man.)was leaked online. What I found interesting is that both he and Jack share the same hands.
Anyway, my theory is that Spirit Halloween commissioned Gemmy to make a life size Jack, dressed in his "Sandy Claws" outfit, but the pumpkin man was leaked as while the actual life size is kept under wraps. Spirit is very protective of their animatronics. If memory serves me correctly, someone in the Gemmy community was threatened with legal action by Spirit for leaking their props too early.
Back to the pumpkin man, it seems that the Nightmare Before Christmas audio was chosen at random, but I don't know.