Are all Magisk Modules Open Source?
Github does not equal open source. Because a Magisk module is on Github it does not mean it is open source

You can have both open and closed projects hosted on Github and Magisk is no exception as you can see from this XDA post by Magisk developer (though instructions pertaining to Magisk are outdated)

There are projects hosted on Github with a portion of the code which is closed source and is offered as paid or pro version, for example code of
FairEmail is available on Github but the pro version features are not open source. Though I am not aware of any such Magisk module with pro component

Though this SO question is old, it gives pointers to better understand