Then you have done something wrong.
After copying the camera-ui to /usr/bin, and before rebooting
check that the file is indeed there

ls -l /usr/bin/camera-ui

and that the camera-ui is working

The reflash wasn't necessary:
Just in case this happens again.
set no-lifeguard-reset with the flasher utility:
flasher-3.5 --enable-rd-mode --set-rd-flags=no-lifeguard-reset
This will prevent the reboot loop.
Then, fix the camera-ui binary and after that remove the flag:
flasher-3.5 --disable-rd-mode --clear-rd-flags=no-lifeguard-reset