No, they are also mined. However, all signet blocks are signed, which means, that only developers can mine them. Of course, you can make your own signet, and then it depends, what challenge will you put as "signet challenge", which will then be required in every coinbase transaction.

So, signet is just "testnet with restricted coinbase", where you have to sign your block, before you start mining.

Edit: https://en.bitcoin.it/wiki/Signet