The trackballs are listed as "completely open-source" on their website, but the mouse is listed as "open-source firmware". This is consistent with what's available on their GitHub page—the Altium and STEP files for the trackballs are available on their GitHub page, and the firmware for all their products (including the mouse) has been upstreamed[1].
[1] https://github.com/qmk/qmk_firmware/tree/master/keyboards/pl...