From that page:
"With the release of Windows 10, we added Fullscreen Optimizations – which takes full screen exclusive games and runs them instead in a highly optimized borderless windowed format that takes up the entire screen."
Later they give solution how to disable it for specific game but in case of GW2 it doesn't fix the issue. Even with “Disable Fullscreen Optimizations" option turned on in Compatibility tab - game still runs in windowed borderless instead of true full screen.
Again - this is not the issue with DX9 renderer - that's why I put post about it here in beta graphics issue thread.
If I select "Full Screen - 2560x1080" in DX9 mode - game is running at that exact resolution with crisp UI.
I'm pretty sure it's GW2 issue with how they implemented fullscreen things on their end because I'm not having this issue in any other game like Cyberpunk 2077 and Pathfinder Wrath I'm playing right now.
It really does suck since I read that with DX11 there is nice overall boost to fps and even with my GTX 2080ti it often drops to as low as 15-20fps in cities, wvw or big meta events - even with character model limit set to low 😞