Does Bisq somehow use the Bitcoin blockchain for recording trades?

I just noticed that, after accepting a trade to buy BTC for fiat, Bisq says:

Deposit transaction has been published.
You need to wait for at least one blockchain confirmation before starting the payment.

I don’t understand what any of that means. Before starting the trade, I pre-topped up my Bisq wallet with BTC for the security deposit, and it has (and had) six confirmations before I clicked “accept”. So this text can’t very well have anything to do with that.

I want to make clear that I have no practical problem using Bisq, and have used it many times in the past. I’m just wondering what they mean by that text, and whether Bisq actually uses the Bitcoin blockchain somehow. Maybe it’s using automated “smart contracts”?