I have to admit to not paying much attention to development of doublets so I am happy to be corrected :-)
My understanding is that current doublets only achieve this for 2 wavelengths in the visible spectrum and third in non visible.
Yep as I mentioned earlier perhaps the specification should be purely performance based and applicable to all configurations. But for all practical purposes (and still happy to be corrected on the doublet point) my understanding is that this is still only practically achievable in a triplet design.
But from memory the earliest definition of APO came from Triplet Designs which were the only practical way of achieving this.
Apparently it is true that the third crossing in not in the visual but it also apparently true that it still helps bring the colors to focus in the visual. Roland seems comfortable calling a ED or Fluorite doublet an apo, that's good enough for me..
Let me ask you this:
Have you ever looked though a 80mm-120mm ~F/7 FPL-53 doublet at Venus? In my experience, Venus is about the most challenging object in the sky for a refractor. It is pretty amazing, when all is said and done, apochromat basically means that it has far better color correction than the crown-flint achromat that was the standard for 200 or so years.
In my experience, looking though my various FPL-53 doublets I have owned and still own, the way I can tell that they are not reflectors (besides no Central Obstruction): On Venus, they show some chromatic aberration out of focus, in focus, I see none. A triplet would also show out of focus color.
The last time I used a doublet was many years ago....and yes it was a nice scope and I don't recall noticing any CA on any target with that scope.
Now days all I have and use are Triplets ..... my Takahashi TSA120 and TOA150 and i notice absolutely no CA on any targets with these 2 scopes and visuals are exquisite with different spectral classes of stars clearly defined and observed.
However not much point in arguing about any scope out of focus. The point is to bring the best possible image to focus :-)