I think picking up Fultz' option this early is a good sign that they are pleased with his work ethic on his rehab, and that they think he will be a productive part of this team sooner rather than later.
I think it also sends a message to Chuma Okeke that taking a "red shirt" year and signing with Lakeland while doing his rehab will be a positive long-term move for him. It shows the team values that work ethic, and has loyalty to help their players to recover/become the player that everyone thinks they can be.
If Okeke signs a contract, then we will have 14 players on the official "salary" roster, and signing anyone, even a rookie minimum deal will put us over the luxury tax threshold. But having him "defer" his rookie season to next year means we have two spots open on the roster, and can fill both of them with veteran minimum deals without going into the luxury tax.
The bad part for him would be that he could only make about $35K next year and he would be pushing back the end of his rookie contract a year, so won't enter free-agency as soon. On the good side, he would then sign his rookie contract next year at a $400K higher starting salary, because he would fall under the 2020 rules; he would still have the benefits of team doctors, rehab, facilities, and coaching in the Magic system (albeit Lakeland's coaches, but they run much of the same stuff as the parent club).
Truth is, even if he is signed to the parent club this year, he would still spend most of the season rehabbing, and probably start his comeback with limited playing time in Lakeland anyway.