I am sure it is still technically accurate. If I had to guess, Delta restricts the pool of Diamond agents to those with a minimum tenure (i.e. you won't get someone working their first day) and probably requires them to go through a short (<2 hours) computer-based training program to teach them how to handle Platinum and Diamond-specific issues (e.g., applying RUCs/GUCs, co-terminal changes, etc).
That said, those same agents are also in the main queue now so it is only a "virtual" VIP line in the sense that you will only reach a subset of the total pool of available agents.