You want a 4 foot pool with a rubble bottom, making it uncleanable, unwalkable and blocking the main drain?
What would you use it for?

I think they mean once the rubble is backfilled then pour concrete over it.

No you cannot. It must be a suitable fill material such as 3/4" gravel or cement slurry. You will also have to extend your main drains and epoxy in a new rebar floor.Then Shotcrete.