1. Like you said his game is old school and doesn't match up with the type of O they run, its more of a fast paced type.
2. Okafor is now third string center behind Holmes who has shone a better shooting ability and defense.
3. Embiid probably knows that Okafor hates being in Philly and wants out. I don't think it was a diss to not include, rather an acknowledgment that both parties will eventually move on.
Everyone in the league knows the 6ers don't want Okafor and will eventually move on from him so they aren't offering anything that has value. Okafor can bring a lot of value to a team that can suit his style, say a team like Memphis. I think Philly is keeping Okafor around as insurance for Embiid. If Embiid gets hurt again then it's time to make plans for a new third piece of their big 3.