Just solved it. When I ran xdg-open “http://google.com” it showed some warning about kfmclient command not being found. Googling around, I saw that I was missing an env var. I needed:
export KDE_SESSION_VERSION=5
I put that in my ~/.xprofile and it worked.
Should note that I’m running i3wm, but with a minimal kde installation for the pretty windows, dolphin, etc.