The thing is, this kind of online protest is the safest for both initiators and participants. If by "other venues for labour activism" you mean go to the street, you're basically risk sending people to jail (And after the thing, nobody want to risk to get milled down by tanks).
Also, did you know that 996 is by law illegal in China? You can technically report your company if they force you to work 996. But: 1) After that, you will probably get fired for any number of excuses. 2) Your company is clever, they know many ways to get around the law.
Now, after all that dilemma, how could you protected yourself and help other developers just like you? The license idea come out naturally.