I think it is important to distinguish between two important aspects of separating a new fish, Disease quarantine, and Acclimation to Captivity.
Isolating and observing or treating prophylactically to contain the spread of disease is what is primarily discussed. And I think is an absolute requirement of the unlucky reefer. Many lucky reefers seem to get by without it, but as one of the people who from time to time, has unlucky things happen, a perfectly human thing, QT to prevent disease is a must. I have learned my lesson the hard way, losing a whole tank to velvet early.
Unfortunately LA , Divers Den, and most retailers don't have the facilities to adequately guarantee against cross contamination, the responsibility of quarantining disease falls onto the consumer. Not to disparage the retailers out there that do do a good job of this, but research those who do claim to, and make sure you feel confident that it matches known and trusted procedures.
Specifically
Divers Den Page says: "The quarantine period for each fish ranges from a minimum of two weeks..." Which is not long enough for my personal tastes and I would QT for disease from Divers Den. YRMV and I'm sure a Lucky Reefer will be by shortly to tell me I'm wrong.
What good retailers including Divers Den do well is the second part, Acclimating fish to Captivity. Training fish to eat foods, and the artificial environment. Even socializing them with other reef fish,
see this article on the Narcosis Angel that was on Divers Den. Acclimating fish to the artificial reef environment is an often overlooked part of acquiring a new fish that retailers can excel in.