It seems like this Shield is listed as available for a Mage, despite it not being a usable weapon type. Not sure if this is incorrect data on Blizzard's end or.

Interestingly enough, it seems to be the only one at the Cobalt Assembly that has such behavior of the otherwise unusable weapons for a Mage

This seems to be a bug on Blizzard's end as other items like the Farstrider's Buckler also exhibits this behavior:
https://www.wowhead.com/item=23395/farstriders-buckler