Layered with crushed stone for concrete slab, but heavy rain came
If the gravel is that thick it would need to be compacted to keep it from settling over time. It should be done in stages (lifts). Add a layer, tamp that then add a layer and tamp that until it reaches the height needed. An engineer provides this type of info regarding how much gravel at a time for each lift. I know this is not what you asked, but it is good to know.
With that in mind, I do not understand why the wait on the pour since the gravel is so deep, and the contractor is just using that as an excuse. Perhaps it will put his schedule behind if he did your job the next day. He is most likely going to the other jobs he already had scheduled for the next few days until he gets a new opportunity to pour your slab