Don't know what to say.
It's been a while since I fired that game up and don't remember having any issues.
I have a screenshot in that format that shows no stretching. I don't remember having any special difficulty with that, unless they updated and changed everything again?

https://steamproxy.net/sharedfiles/filedetails/?id=3159413060

Best I can get right now without stretching distortion is 2560x1440 borderless windowed mode. I've 2 monitors and my primary nicer monitor is 3440x1440 native. The game auto Left Justifies when in borderless windowed mode, so I have 1/4 of my desktop viewable. If I go with the 3440x1440 full screen or borderless windowed, the game looks squished. Now that I've written all this out... I wonder if it wouldn't serve me to make the game display on my 2nd 2560x1440 monitor instead... lol

Ultrawide worked fine in SE for me. Don't remember having to use a mod to fix.