There are different kinds of open-source licenses.
Some “permissive” licenses, like the ISC License, allow you to closed-source a fork or derivative.
Some “copyleft” licenses, like the GPL License, do NOT allow you to closed-source a fork or derivative.
The VESC firmware is copylefted under the GPL.
Of course, if you’re Jason Potter, you just break the law. There’s always the crime option.