Rusty has it right. The 36" diameter is just some advertising copy run amuck. All it is trying to say is the S gauge Lionel flyer trains will run on S gauge FasTrack or original Gilbert track. There is no 18" radius sectional track made commercially so Lionel has not tested items at that radius. They are not carefully bending flex track to 18" radius for product test purposes.
More significant is the change in measurement protocol. When Gilbert made track the 20" radius was to the outside of the ties, so measured to the centerline as we do today the Gilbert track is actually 19" radius give or take a 1/16th of an inch.
