The owner doesn't know so anything is possible and this conversation will not answer Julie's question. I personally would grow grafted trees if I were to start a commercial jakfruit grove but my choice, your choice, and Frankie's choice all being consensus and knowing other growers have chosen to grow seedlings does not allow us to conclude what the original grower may have chosen. Julie can try to find a matching fruit from a different seller that can specify the variety of fruits sold, which can be its own fun adventure, but she may never solve her mystery. Visual identification, like with mango, can be impossible. If the purpose of identification is to grow this variety or seedling, I would recommend asking the owner for genetic material to propagate.