I interpret (and this is only me thinking) that he means in theory that for example you put 7500lbs on the trailer and situate it so that 300lbs of that weight is transferred to the tongue (tow vehicle) that in essence means you only have 7200lbs on the trailer which is the rating of the tires originally mentioned. In theory although highly impractical you could put 10000lbs on the trailer and situate it that 3000lbs of that weight was on the tongue and you would not be overloading the trailer until you jumped in the truck and started to move.
Steve