The 20% rule is not a rule, it is a widely held but really unproven figure, a bit like the 8 glasses of water a day.
And as we have seen from other threads, some people consider that a short obese person even at 20% is considered a worse choice than a normally proportioned person who may be nearer 30%.
If the research done on the horses competing in the Tevis cup is sound, then horses can carry up to 30% in the most testing of conditions without any measurable harm. Remember that these horses will have carried that weight over many miles of conditioning as well as the race proper.
SOME horses will only be able to carry 20%, some more some less.
I also refute that horses were designed to pull, I do not believe that either God or natural selection designed the horse to do any more than graze and try not to get eaten. Once man became involved in the selection process we then produced horses to pull, to run fast, to ride, to carry knights in armour, we also produced the halter horse which appears to have no function apart from look pretty. Lots of different shapes and types of horse, some better at being ridden than others, some better at weight carrying than others.