typically firestopping and angles are only required around a duct opening when the duct is passing through a 1 hr rated assembly. when you pass through a two hour rated assembly a fire damper is needed. most likely the residence is wood construction and the walls and floors are not fire rated.
I wonder if it is referring to a supply or return plenum in an effort to make sure combustable floor base is used in a downflow position or an upflow where the return is cut into the floor below the furnace.
I usually see it in upflows located in the basement where the return uses the floor joists to link the return trunk to the inside wall via cutting the baseplate (and topplate for the second floor). Register is then placed on the 1st floor drywall and the wall cavity becomes the return. St. Louis public works can't tell me if it is code or not...they don't bother with resi. I guess it's okay because wood floors aren't fire rated like the previous poster noted.