The very dimensions of the (octagon) Carlon J-box were selected for ceilings. Octagon boxes were originally crafted for ye old classic porcelain Edison socket style outlet. (key-less or switched)
Subsequently, other light fixtures were dimensioned to directly affix to said octagon J-boxes – or to enclose an adapter plate for same. (spider ring and/ or lumière strap)
The additional weights involved caused the NEMA players to come out with up-rated designs that could handle ever heavier lumières. These became known as “hanger boxes.”
In the latest incarnation, hanger boxes have been up-rated to “fan-rated.” Not surprisingly, considering the price premiums and the usage, all such boxes became known as ceiling boxes.
That's a thumbnail history of the octagon box, and its (round, ie Romex-BX era) sisters.
[ The octagon provided flat surfaces facing side-ways in an era when piping was common – all of this before BX or Romex. It's that old. ]
If I was piping towards an Edison socket, I might still use an octagon box.(metallic) But, if you visit any big box store you'll notice that octagon boxes are (now) thinly stocked. (metallic or plastic)
Any ceiling rated box is also sure to be rated for (less demanging) wall usage. You stumped the tech rep.
I have to say that in my humble experience, pancake J-boxes are the universal norm for stucco exteriors. Hereabouts, stucco is always applied to OSB/ plywood. (exterior grade) So it's no big deal to punch a hole after the sheathing is up and slap on a pancake – which can be placed just about anywhere to suit and at great speed, certainly far faster than setting a box that requires punching a big penetration with a hole-saw.
(The carpenters have finished the rough-in, setting the box and penetration is all on the electrician. The stucco contractor is expected to mesh-in around the pancakes and slather on his green coat, etc.)
Pancakes beat octagons, hands down. Again, these are terminal boxes that only feed a sconce.
The very dimensions of a pancake make it automatically perfect for exterior stucco.
Lastly, my practice is to always screw such pancakes down. Nails are shunned.