I wouldn't be so sure. China is at the world's forefront of automated techniques to be able to spy on and manipulate people through their own devices at massive scale. If they had some semi-workable technology to fingerprint individuals through their typing patterns, in conjunction with fingerprinting the devices they were using through other means, that would make perfect sense to me.

I don't think it is especially a concern for Deepseek specifically, for reasons discussed elsewhere in the comments. That one particular aspect of the privacy issue is probably being overblown, when there are other adjacent privacy and security concerns that are a lot more pressing. Honestly, that one particular detail isn't really proven simply because it's in the privacy policy, and even if they are doing something like that, its inclusion or not in this particular privacy policy or this app isn't the particularly notable part about it.