Agreed. Open source software still allows for paid use for particular features. Typically repos on GitHub and/or other sources just skip hosting releases on that site (in the case of fully unlocked software that lacks 'licensing' code). This is somewhat common on Patreon, where the source code is on GitHub, but the creator hosts official releases on their Patreon page.
Sure, a savvy user can build a release themselves, striping out the licensing, but only a savvy user will know how. The majority of Windows users will continue to use the standard free version, or pay for BigBox, I imagine. Linux users may be more knowledgeable on average, and thus capable of building it themselves, but I have two counter points on that:
1. Linux community is small, so even if 'rogue' users build a release for themselves, the impact on existing Windows user licensing income should hopefully be negligible.
2. Linux users are often more aware of the license terms they are agreeing to, and what all goes into software development. I'd be willing to bet that the majority of Linux users who want access to BigBox will be willing to pay for it, out of respect to the dev team.