It literally doesn't though. The wiki page is for copying tmux copy-mode's internal selection. The selection and copying mechanisms of the terminal emulator still work just as well.
I think it's undocumented but you're supposed to be able to have update-environment set to '*' so it always uses the parent shell's environment variables when creating a new session.
I have a very quirky setup with tmux and rely on this feature, but it's broken IIRC and add important variables to update-environment manually.