I have two questions:
- Can you set that resolution on the desktop?
- Does the game support borderless windowed?
If the answer to 1. is yes, then it should work in borderless windowed as well.
If it doesn't work ingame but only on desktop, then the game either isn't setting the window parameters correctly (if it's actually borderless windowed) or it's actually just exclusive fullscreen.
Lots of games nowadays don't even bother naming their modes properly anymore so "Fullscreen" now actually means borderless windowed.
Also I recommend DXVK. Just try it for giggles, it may magically fix the issues.
(Also that res isn't 21:9, that's even wider, because the closest to 21:9 would be 2560x1080, but that's a minor nitpick)
From what I've been able to understand, FH4 (so I assume by extension FH5) don't use exclusive fullscreen at all, meaning that the game is technically running borderless already.
Can you try, just as a jank workaround, open the game as usual, go to windowed mode (press alt + enter) and then resize the window to span across the screen(s)? If it works properly it should keep the proper aspect ratio and resolution.
The only thing that comes to my mind is that the game just refuses to set the res probably because it detects Intel graphics.
I'd look for a config/settings file and set the resolution manually in there. Maybe that's the only way.