The Eitz Yosef explains that תחית המתים is not going to be יש מאין (a creation from no other source than God). There is only one creation that was יש מאין. Everything else is יש מיש, a creation from something already existing. Even תחית המתים will be from something. There is something that God made that doesn’t decay, but will bring about the body in the time of תחית המתים.
Maybe it shows that in the creation of man, was the blueprints for the time of תחית המתים. God created in man the seeds of the future.
[Like when you add cat 7 wiring into a building. Not for now, but for the future. ]
Man is designed in a certain way that is that he has the capacity to flourish in the existence of תחית המתים. This לוז in specific is manifest this point that we are designed to exist in future existence.
Since man in general will decay, this is the nature of the body, but yet we see there is the capacity built in to be able to survive this decay.
The main point is that there is a manner in which we can see that the body can come back. Whether there is an actual bone or we are talking about something else such as DNA or some other thing, the main point is that there is a something that supports the idea of תחית המתים. This is what ר' יהושע בן חנניא conveyed to אדריאנוס.
I want to draw out one additional point that may be obvious. It would seem that the premise of the Eitz Yosef that God will only create יש מיש, from that which already exists, sets the stage for this concept. In other words, since God will not initiate the process of תחית המתים from nothing, then it must be that there is some element of the body that remains that can be utilized in the process of תחית המתים. This element is what the luz bone represents.