Correct, there is no such thing as "Class III+". That's why I put it in quotes and followed with "if you like".
(Writing...so prone to error in transmission...I should use more icons I guess.

)
Who at Toyota said it is a Class IV ? Most of the time when I contact front-line Customer Service people, they don't know much, and they also unfortunately have the tendency to make stuff up where they don't know. (Like many dealer employees - see my rant in greynolds thread...)
Common sense: I really doubt Toyota would put a 10000lb rated hitch on if the vehicle is only rated to tow 6500lb. There is no benefit to Toyota or the customer. There is, however a cost penalty to Toyota and a weight penalty (with all that entails) to the customer. The added weight might also reduce the fuel economy and performance figures - OEMs are always trying to figure out how to reduce vehicle overall weight.
Naw, it's prolly just a 6500lb hitch. Nothing to sneeze at, mind you - the 6000lb rated Reese I put on my LX is quite beefy, and probably underrated anyway.
But as far as towing more than 6500lb with a 5500lb vehicle: I wouldn't even want to exceed the 6500 limit, because doing so could result in a load exceeding the weight of the tow vehicle - a dicy-sounding situation. Could lead to "the tail wagging the dog", something to be avoided at highway speeds, eh?