They're thinking of things that see a lot of gasoline. The smear that is between your flanges won't be troubled. Most high temp sealants I've seen specify that, and they've been happily used on crank cases etc.

RTV stands for room temperature vulcanising. It just means that it's a silicone sealant that once you get it out of the tube, it starts to set. That's a good property for silicone sealer to have.