Techtastic found this information within the User Agent Profile for a phone on Samsung's website. A User Agent Profile is a standard maintained by the Open Mobile Alliance that provides information about a device's specifications so that content providers can produce content in a specific format that fits that device.
These profiles are usually stored in a database run by the device's manufacturer, according to Developers Home, a website with basic information and helpful articles for developers. In this case, it seems like Techtastic was able to get into the Samsung Mobile database.
The phone mentioned in the User Agent Profile has a model number of SM-G920A. This doesn't guarantee that it's the Galaxy S6, but Phone Arena speculates that this could be the AT&T version since the Galaxy S5 for AT&T has a similar model number (SM-G900A).
Here's a look at what's in the User Agent Profile:
The idea that the Galaxy S6 will feature a 1440 x 2560 resolution screen isn't new, but the fact that we're seeing this specification appear on a Samsung Mobile website makes it seem much more credible. The Galaxy Note 4 has an AMOLED screen of the same resolution, and it's gorgeous. Display expert Ray Soneira of DisplayMate crowned it as the best overall screen on a smartphone, and it's possible we'll see the same technology on the Galaxy S6.