that just means one has been tested as such, whilst other may not have been. Cowstails don't absorb much load, that is far more reliant on the rope above you
TLDR: people can choose to use the Evolv Adjust as a pair of cowstails if they understand the risks, but they should be aware that it is not designed for this use and the rope used is too weak for it to be rated for such use.
From the Petzl instructions:
"
1. Field of application
Adjustable positioning device for aid climbing.
Warning: the EVOLV ADJUST positioning device is not PPE. It is neither an adjustable lanyard (EN 358 standard), nor an energy absorber for self-belayed progression on a via ferrata (EN 958 standard), nor a lanyard for work at height (EN 354 standard).
Be sure to use a belay system made up of personal protective equipment (PPE) when engaged in activities at height.
This product must not be pushed beyond its limits, nor be used for any purpose other than that for which it is designed.
"
Note that all diagrams showing practical use of the Evolv Adjust include a separate belay system.
Also: "7. Precautions for use. Do not fall on your EVOLV ADJUST."
I assume it uses the same metal adjusters as the Connect Adjust, but it does not use the same rope. I believe that, if it _was_ tested as PPE under an appropriate standard, it would fail because the rope (actually accessory cord) will not meet the required strength.
That said, I suspect it is unlikely to fail under any realistic load... I personally wouldn't want 8mm cowstails, but I also wouldn't suggest it was totally unreasonable for the informed caver to make their own decision about. It is also (assuming the metal adjusters really are the same) a cheap way of getting two, rather than one, of the metal adjusters. You could always replace the 8mm with 9mm rope to make a more conventional pair of cowstails.