The insurance company says the vehicle was repaired properly and has thus not suffered diminished value, is that true?

Most certainly not. The insurance company is telling you that because they do not want to have to pay the claim. An inherent diminished value claim is based upon a vehicle being properly repaired. The vehicle is still worth less because it is not what it was before the accident. A brand new vehicle is not made brand new again by repairing it following an accident.