No, it's not only a FireWire connection needed. The manual also says this: "With Windows, the RML-series mixer uses a PreSonus FireStudio ASIO driver, installed with UC Surface, and shows up in the Sound control panel as a PreSonus FireStudio interface." This article from the Knowledge Base should explain it all. If you search for "audio firewire", you find more articles.
Please note that the FireWire returns are normally routed to channels 1+2 of your mixer, so you need to select FireWire as input. Part B) of the article describes how to change the routing to the "digital return" (Dig Ret) stereo channel. There is NO processing on the digital return.