Recognizing that there are paths available other than "doing what the company says" or "walking away" is a good thing, and people should speak up if they feel their work is being used in unethical ways.
There's a view that employees are subservient to employer's wills, to the extent they agree to stay under their employ. While obviously true to some extent, it seems silly to extend this to carte blanche for what employees must agree to be complicit in.
Leaving is step 2. If the employer continues to make choices that don't align with the employee's morals, the employee should leave.
For employees at a company like Google, both are important. Google is massive and impacts many people across the globe. And the employees are extremely employable - they could leave and go elsewhere with little personal sacrifice.
In this case investors are who can make any worthwhile difference and they should be appealing to investors. However, I imagine a significant chunk of investors don't even know/realize they are due to whole/broad market funds.
Google sees 17% of the world's population as potential customers via China so who is going to have their attention more 'thousands' of employees or 1.4 billion potential customers? The only way they're going to get Google's attention is to get shareholders concerned. There will always be employees willing to be on board with this, fact of the matter is most people just don't care as long as they are getting paid. They need to get investors against it if they want to have any hope of getting Google to not do this.
A Medium article, or a petition, or a 'hey thousands of us think you shouldn't do this', is unlikely to do a damn thing. If they want to make a stand they should have Amnesty International create a video campaign that describes, FROM PUBLIC SOURCES (to avoid any potential blowback from sharing company secrets/violating NDAs), what Dragonfly is and have non-Google employees that are industry experts comment on why they think it is a bad idea. Then the Google employees should share this on all of their social media, share it directly to their friends. You want to get outsiders championing the cause and generating attention and you want to minimize any single employees risk as much as possible by using non-employees to be the public face.
If I were doing it I'd do a video as Amnesty International, try and get the Internet Freedom Foundation on board, reach out to college human rights groups and try and grass roots the shit out of it to get as much media attention as possible which would increase chances of reaching the eyes of investors.
Doing it as a group of employees though, ehhhh, that's almost certainly going to do nothing and if anything it'll have a negative impact on employees and potentially result in future policies designed to thwart future attempts like this.
It sucks because it's too big of a market to effectively deter Google from wanting to bend to the will of the Chinese government.