If your present mc oven is not vented to the outside, then there is not going to be a vent in place to vent your new mc. I suspect AS will tell you just to install it as is. I agree. I don't think venting is really needed for a mc oven, even if it is a convection type. Venting is more for stuff that you burn on the stove. Granted, the TT space is small, but then you have all those windows to open up to take care of the problem.
Additionally, installing a vent is another opening in the roof for rain, hot air, cold air, etc. to get into the TT.
Just my .02.