Since MSS are unlikely to tell us their decision making process it is quite opaque. It could have been CCTV, an informant, an unfounded denunciation, something they said on WeChat, one of the main compromised Chinese apps. It really isn't open to her claim this level of confidence.
If the keyboard is leaking keystrokes or word searches on a wide basis it would be difficult to hide technically. DFIR techniques for this are pretty straightforward, I'm sure plenty of people in HK could do it. Why no details?
But ultimately this is a much bigger Android problem, and won't be solved by fixing the keyboard (which OWS is obviously unqualified and ill-equipped to do). A broad ranging device lockdown guide, and OPSEC training (like [1] but for protest groups), is necessary to have anything except illusory protection. I don't think OWS should get into the business of issuing security advisories for all the platforms that they port to.
The pro-democracy groups seem to have this stuff figured out as well as you can and still have a visible protest movement. Very much following Chairman Mao: "The revolutionary must swim with the fishes."
[1] https://www.slideshare.net/grugq/opsec-for-hackers
