It's definitely doable (assuming their math checks out).

What I can gather so far is they base it on a variation of the zero-knowledge proof - some mathematical magic that allows you to prove that you own a secret in multiple ways, thus decoupling one check from another.

So you mint an off-chain coin and then spend it again, this time revealing the secret, so there are no double spends.

It's like having a special challenge-response chip. No point in saving challenge or response and no way of linking several challenge/responses to your chip. Once you spend it, the "chip" is destroyed.