Most likely, they still won't know whether it's evidence or not, even when they've finished processing it.
If the shell matches the one left at the school, all they will know is that it matches the one left at the school. They don't know that either one of them actually came from the murder weapon. Nor will they know whether any other evidence found in or on the truck is from the shooters or from some other poor sap who rented the truck.
The only real clues they will have is the rental information. They can try to match the handwriting to any other samples of handwriting they may have (tarot card, piece of paper with directions, etc.). If the rental information turns out to be a false ID, they can figure they may be on to something. If the rental turns out to be to a local who appears otherwise legitimate, then this may be more misdirection by the shooters.
The one exception to this that I can think of is that the shooter may be in for a real surprise if the truck had some kind of GPS that recorded a record of where the truck has been. I would bet that most rental cars now have some kind of transponder that can be activated from the rental counter to send out a signal to help locate it if the rental is stolen, so there may be some kind of data maintained in the truck itself.
I can understand Moose saying they need more time to investigate, run tests, etc. before jumping to conclusions. But I see no reason why he couldn't have said "Yes, this shell matches the one left at the school. But we still don't know if it came from the murder weapon or whether it was planted in the truck to try to confuse us."