For what it's worth, this appears to be an active dongle using a MegaChips device to do the DisplayPort to HDMI 2.0 conversion. USB Type-C alternate mode handles the passing of DisplayPort audio/video data from the GPU over the USB pins (ML0-3), and the MegaChips device converts that to HDMI 2.0 TMDS signaling.
No, my Google-fu also failed me. There's also an SPI ROM that is an input into the DP-HDMI2 chip, so there's likely firmware there that is not part of Google's open-sourcing.