My guess is that the shield you picked up is a modded Fabled Tortoise that removes the movement speed restriction that a normal unmodified Fabled Tortoise would impose. I found a YouTube video that is a bit grainy but shows a Legendary shield named only Blast Proof. FYI, the Fabled Tortoise is legitimately available in a Blast Proof variant.
As for being “one of the few false items that stays”, I believe the other player was referring to patches removing certain modded items from the game but not others. The item you picked up was likely one of those not removed via patches. As others have stated, your best bet is to toss it. If you need a pretty good legitimate shield, you could always purchase an Antagonist in Flamerock Refuge assuming you have Seraph Crystals and the Tiny Tina DLC.
EDIT: A further search yielded this find on reddit from 2 yrs ago. Confirms my suspicion about the movement speed reduction being removed. Also, notice the particular wording of the 1st response by corpseflakes. Perhaps I found your “guy”, OP 
Forum ID: Poisd2Strike
GT: Poisd2Strike
Trades: http://forums.gearboxsoftware.com/t/poisd2strikes-humble-shop/367700
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