I highly doubt it will be cannibalizing any JL sales. Besides some shared design language I bet the JT is going to be a legit midsize work truck with average off roading ability. Most JK and JKU sales go to those who want a more rugged SUV and not looking for a pickup truck with a flat bed.
IIRC they have said that the design and everything is done on both the pickup and regular JL. They just need to build it. So they could show it at the same time? Probably wont just to draw out interest and hype tho.