Well of course it's marketing, lol.
It's also true, on those two specific PSUs. It can't be compared to others and remain accurate though.
As a random example, the Tt Toughpower XT 1275w in a decently cool ambient won't even turn the fan on at 600w. The NEX1500 will sound like a turbine at 600w.
Essentially, it's pretty solid testing and true, but only for those PSUs tested. Applying the results of those two AXi PSUs to any other PSUs will result in issues.
Odds are halfway decent that the be quiet! Dark Power Pro 850w is quieter than both of them at 600w.
Speaking very generally, a higher wattage PSU of a given series will have larger heatsinks, which in turn require less airflow to get rid of a given amount of heat.
It falls down a bit when the two PSUs are different designs with the same badging (as in the example in the blog), and more so when the fan control sensor is attached to different things.
The thing to remember about corporate blog posts from trusted people is (in my opinion, of course!) that while the person may only be willing to post correct -truthful- data, the owner of the blog can choose whether anything is posted or not.
If Johnny's data on those two PSUs hadn't proved the point that Corsair wanted proved, that blog would not have been posted. Corsair probably would not ask him to make numbers up, and I doubt he would even if they asked him to.
Please do note that I know absolutely nothing about internal Corsair activities. This is my take on generic corporate actions based on my personal experience.
Anyway, the short answer is, it's true for those two, but not necessarily any others.
