Thanks!

It is not a coinjoin and there are four transactions: (1) the buyer deposits into Swap Address A and (2) the seller deposits into Swap Address B. Then some off-chain communication happens to enable each party to sweep the other's money. Then (3) the seller sweeps the money from Swap Address A and (4) the buyer sweeps the money from Swap Address B.

When #2 happens the buyer can check whether Swap Address B contains the output of a coinbase transaction. If it doesn't, he can just abort and sweep back his money from Swap Address A after a timelock expires. Note that my software does not currently check if the utxo being sold is the output of a coinbase transaction, that is currently up to the buyer to check. (Fixing that is on my to do list.)

Right now the software does not check if the seller is a miner so you can use it to sell your doxxic change. But even when it does check if the seller is a miner, you still might be able to use your doxxic change to buy a coinbase utxo from a miner. It's up to the miner how small of an output they'll create. So if you have a doxxic change output of, say, 15310 sats, a miner might go ahead and do a swap with you.