I suppose one should respect Barnett's, but I don't buy it. If cage length is the determining factor, then why isn't the long cage version of the same derailleur rated for a larger cog size? Likewise, why is a MTB RD with the same cage length rated for a different max. cog size?
I believe that you are mis-reading a rather poorly written section.
The manufacturer’s rating is based on an assumed derailleur-hanger length.
True enough. But wrap itself is not the
sole determining factor. Nor is it the major factor. They are saying only that wrap is
a factor.