The character likely has not logged in since Pandaria release. Currently achievements are a bit buggy, such that eg Loremaster and others are randomly re-awarded upon login etc like I am sure many have seen happen since Pandaria release. I would suggest that this is indeed the case here : the character is a Scarab Lord and rightfully has the mount, but the BoA achievements are bugging out on him since he has not logged in since Pandaria release.
You can see that in his achievement history he has randomly been gaining achievements. Now, these achievements are very obviously earned on a different character on the same account. It stands to reason that the achievement system right now is borked and does not properly update in cases such as this.
Also, just to make sure: The mount is not available ingame, on the BMAH or otherwise.
You will see that the only FoS he has is the one acquired through SC2 and his Black Qiraji Battle Tank is not showing up either. In fact, he seems to have no mounts whatsoever beyond the store mount. Since he has not logged in.
If I logged him in now, I am fairly certain that the Armory would update and show it properly. Or maybe not. Who knows at this point.