What I expected - that if I would want to install Openbox - then Xorg will be forced.
I typed:
# pkg install xlibre
... and X11 XLibre implementation installed and Openbox installed properly and 'xorg' packages was not forced.
The binaries of XLibre still have the same old 'Xorg' names like 'Xorg' is the server binary name (not 'XLibre') and xinit(1) is still xinit(1) ...
But if you already have 'xorg' package installed and you would like to install 'xlibre' package then there would be conflicts as they install file into the same places - and often with the same names.
Hope that helps.