JP was the man who made the 911 call, but he mentions in the call that he was alerted to the car's presence by a neighbor, who was called by another neighbor and told of its presence.
So, would this scenario be a possibility:
The first neighbor (let's call him neighbor A) saw/heard the car around 11ish, but made the assumption at the time that maybe it was neighbor B or ok'd by neighbor B, so he let it go without investigating.
But then, when he sees a car still out there in the morning, submerged, he calls neighbor B to check if it belonged to them or knew anything about it. He doesn't know anything about it. So neighbor B calls JP to see if he knows anything about it, but JP knows nothing about it, and finally calls 911.
So, could it be it was this unknown neighbor A who gave this "very good information" to police?
Possible?
Yes. I think that is very possible.
Whatever the reason is that prevented neighbor A from calling 911 himself, also prevented him from calling when he saw/heard the van around 11 pm. We've always wondered why neighbor A didn't call it in himself. The above scenario would explain that.