The statement is inexact. The exclusive fullscreen mode does work via Vulkan, but it is buggy. It gets automatically reset to borderless window at the refresh rate of your desktop (usually 60 Hz) every time you change a graphic option or you restart the game. You can easily reproduce this by selecting a 50 Hz refresh rate and the display will switch to exclusive fullscreen at 50 Hz, until you restart the game.
Topic on Talk:Red Dead Redemption 2
I did additional testing at your suggestion. I created a custom resolution with the width and height of the native and set it to 50 Hz. I launched the game in the DX12 mode but the game window was borderless. Then I set the API to Vulkan and restarted the game manually - after this it still would not minimize to the taskbar from the launch screen, no settings touched. Then I looked at the settings to confirm that the Screen Type was Fullscreen. Setting back from 50 Hz to 60 and back and restarting had no effect either.
Keep in mind that I do not have a G-Sync monitor, but the statement we have in the article isn't exclusive to G-Sync monitors either.
I stand corrected. After several tests under both Win7 and Win10, I came to the conclusion that indeed the exclusive fullscreen mode is buggy and will silently fall back to borderless window mode (while still indicating "fullscreen" in the settings).
The difference is that in Windows 7 the game can force the desktop to change its refresh rate (for example 50 Hz), but for some reason it doesn't work in Windows 10. Under Win10, if your desktop is 60 Hz, the game will be locked to 60 Hz (borderless) even if you try to change the display rate in the settings.
What's also interesting is that this "refresh rate" setting is actually controlling two options: the display rate proper, but also the frame rate. Under Win7, when you set 50, you actually set 50 fps as well as 50 Hz. If you try 50 under Win10, you'll get 50 fps BUT forced 60 Hz, which will produce obvious stuttering.
In short, exclusive fullscreen doesn't work. And if you want to run the game at anything other than 60 Hz, you have to change your desktop at the desired display rate before running the Rockstar launcher. Of course that doesn't help much for GSync/FreeSync screens.