If you send BTC for XMR, and then XMR for BTC that should be considered mixing.
Well, Monero is explicitly listed by Theymos as "not a mixer", when it comes to signatures. And also this point:
2. It is possible for the mixer to steal property passing through it. Assume that the sender does everything as correctly as possible. Also assume that no miners/verifiers on the base-layer cryptocurrency are evil. But assume that every other actor involved is evil (everyone able to vote in a DAO, every coordination server, every counterparty, every member of a multisig, etc.). Ignore short-term software bugs which are expected to be quickly fixed.
As long as BTC/XMR swaps are decentralized, and as long as Bisq is decentralized, it can be freely advertized here.
Which means, that it can be a mixer in your eyes, but it is not a mixer, when it comes to the rules of this forum.