bullshit, light is as good as usiing exhaust gas to tell if there is a leak or not.
Its quite a common technique to use to verify leaks or not, you dont even need to be in a pitch black room, if you cant see light coming through, then its not leaking, you could use water too which is prob a little more verifiable however not too many people are keen on pouroing water through their turbo.
the job is to shine a bright light through the turbine housing, and see if you can see the light coming through area's it shouldnt.