hmm, but I don't understand how withdrawing any cutting tool under power can dull it, otherwise drills, boring bars and endmills would end up dull too, right? They all cut holes, they all have cutting edges in contact with the hole wall during retraction, just like reamers. The cutting edge is facing the work in the same way, but should be cutting less or not at all on the withdrawal. The only way I can think of dulling a cutting edge on withdrawal is if the reamer is spinning too fast and rubs or if the very shallow depth of "recut" leads to work hardening. No reason why the latter couldn't happen with the spindle off, akin to stoning a cutting edge.
Not looking to pick a fight, I just can't understand the physics behind it nor have I found any other mention of it on other machining websites. When mentioned, the only caveats were: spin slowly/ feed fast, don't leave too little to cut or the reamer will rub, don't reverse the spindle or you'll dull the reamer, don't stop the spindle or you can leave lines in the bore.