I've not watched, but sounds likely to be inconclusive, ultimately.
Not a fascination for me, but find intelligent examination of cold cases interesting and especially with the contemporary level of forensics available to fully establish long held evidence. Some interesting cold cases in US have been illuminating with the consequences of larger database resources in dna, making much more valuable research method far reaching.
Not the case here, it would seem. Watching a famous case down here on South Coast, recognised a police officer friend in the documentary, who was ultimately responsible for acquiring and conserving the vehicle of suspect, before planned scrapping. That to give conclusive evidence in conviction through very detailed forensic capture and examination.
In these cold case, its no surprise the resistance of suspect, and strength of that in denial, until faced with indisputable forensic analysis. Sometimes, even then there's absolutely no acceptance of their part in the crime.