"supposedly fully backed" isn't the same as "actually fully backed". If they are being created out of thin air, it would mean the claim about them being backed by dollars was a lie.

Edit: Although, whether or not they're backed by anything isn't really all that relevant, considering that their terms of service[1] say: "There is no contractual right or other right or legal claim against us to redeem or exchange your Tethers for money. We do not guarantee any right of redemption or exchange of Tethers by us for money."

So backed or not, nobody has any obligation to give you dollars in exchange for your tethers.

[1] https://tether.to/legal/