There doesn't seem to be any justification offered on why it's not yet open source.
Not that one is required, but it is suspicious that such a commitment isn't a priority, and that maybe there is a desire to keep closed-source as an option.
Personally, I'd not want to check any assets into such a tool before it becomes open-source.
Open sourcing code can require a lot of resources to do it right (manage the community, handle licensing etc). If you don't have customers asking for it and you don't need it for customer acquisition it might not make sense.
Personally, I'd not want to check any assets into such a tool before it becomes open-source.