This mod doesn't really add phpBB2 into pligg, it simply logs you into phpBB automatically when you log into pligg. Same with logout and register.
This has been added to the SVN for Pligg as a module. It's disabled by default and if you REALLY don't want it you can just remove the whole module folder.
It's also still a work in progress. If the user already exists in phpBB and the passwords don't match you won't get logged in and currently changing your password in pligg does not change the phpBB password. Plus it still needs a little security work.