But don't the Cyclo standard and Oppy derailleurs (which use two jockey wheels) precede the Simplex by a dozen or more years?

I have no first hand experience with the Simplex TdF, but the similar derailleurs I've worked with --Cyclo Benelux and Resilion-- are extremely difficult to work with. They shift quite well when set up well, but adjustment is prohibitively difficult. It is easier to respace the axle, by moving washers around and that kind of thing, than to adjust the lateral travel of the derailleur. These derailleurs taught me to patch a tube without removing the wheel from the dropouts, how's that for a recommendation!

When you compare any of those --Simplex, Huret, Cyclo Benelux, Resilion-- to the Campy Gran Sport, it is obvious that the Gran Sport was a game changer. Regardless whether this was evident at the time (and I can only guess it was), six decades' worth of derailleurs have built on the Gran Sport design while the pull chain derailleurs have been obsolete for five of those decades, if not all six.