I think the author is being clear enough: right now it's proprietary freeware, and the degree to which it'll be open source will be decided later.
Wonder Unit even has developers sign a CLA agreement assigning all contributions to Wonder Unit (https://cla-assistant.io/wonderunit/storyboarder)
Maybe the author should consider a more explicit proprietary for liability purposes and clarity about revocability, exclusivity, transferability, et al, but for now any user should assume he can choose to do whatever tomorrow and proceed with caution.