Unfortunately, once a free weekend was played steam has data of the game saved on your library. Which keymailer finds and marks the game as owned. For it to unmark it, we would need to get access to steam and delete the leftover data. We obviously do not have access to steam, which is why we cannot do anything from our side.
Shoot an email to the devs and let them know! You can reach them here: firstname.lastname@example.org