No account, no email, no documents. Here is what that buys and what it does not.
There is no registration because there is nothing to register. The note is made in your browser, the deposit is a transaction you sign, and the withdrawal proof is built locally. At no point does this service need to know who you are, and there is nowhere for it to put the answer if you volunteered it.
Your deposit and withdrawal are both on a public blockchain. The point is that they cannot be joined, not that they are invisible. This site is served over the internet, so the server sees request IPs the way every server does. If your threat model cares about that, use Tor or a VPN — and be aware that a mixer telling you it keeps no logs at all is telling you something it cannot prove.
Deposits are held briefly before entering the pool so they can be declined and returned, which means the operator sees the incoming transfer. Once a deposit is in, the zero-knowledge withdrawal hides which note is which from everyone including them. The residual risk is a thin pool, and that is a number, printed on the pool picker rather than buried.