1.009 is allowing for the error in a hydrometer. 1.011 is effective if it is monitored correctly with a refractometer.
I'm having a hard time understanding this. From my experience all hydrometers read differently. My current IO hydrometer reads about .002 higher than my previous SeaTest hydrometer. My LFS
refractometer reads about .004 higher than my old SeaTest hydrometer. Because hydrometers are notoriously inconsistent and inaccurate, I would suggest only using the hyposalinity treatment if you have a well-calibrated refractometer. If you can only get your hands on a hydrometer, use another method, such as copper. HTH