User blog comment:IamJakuhoRaikoben/Chapter 346: The Law of Retrogression, Review./@comment-16344653-20130802171121/@comment-7418318-20130802171548

Not necessarily. Human subordination magic doesn't reduce the magic power of the target, thus Celestial Spirits are completely unaffected.

But Retrogression does reduce the target's magic power, which would also mean that the summoned spirits are also weaker. The strength of the spirit depends on the summoner's magic power after all.