Well in this case such a voting would be done using some variation of a ZK-Snark (or some other cryptographic algo) which would allow you to check your vote counted, yet remain anonymous.
In our current system you can find out that I voted, but not how I voted. That should be how it's setup.
That said, We're talking about digital voting here (or at least I was) and the comparison is to a digital centralized solution, in which browser history, cookies, server logs, and not to mention the database itself has issues that can compromise the voter and the election.
The key system, wouldn't necessary have to work that way (you could have a derivative id or something like that, the system could automatically assign tokens each election without the need for manual distribution, you can throw away the temporary key, etc..). It's a open problem though, and it's good to bring these critiques up and think about solutions, I woudln't dismiss the whole thing though on the first problem found, it's easy to criticise but most technologies do have issues initially that need to be solved (and that's part of the fun). What's so interesting about decentralization technologies is that they open new possibilities and novel solutions to old problems.