Just empty words unless you got the big stick to back them up. Otherwise you just end up looking weak only able to protest without being able to take punitive action.
That’s the dilemma Beijing faces.
I am sure Chinese state security and counter intelligence would have plenty of evidence of direct US involvement in the HK riots at a fundamental level.
Publicising such evidence might embarrass the US a little, but what would that achieve?
Most of the brainwashed sheeple in the west would automatically chalk it up as fake news, and anyone inclined would give such news an objective look would probably not be surprised that the US organised the whole thing to start with. And have you see Trump? The man is embarrassment proof.
OTOH, having all these evidence out in the open would massively rile up the Chinese public, who’s anger could easily turn against Beijing if the government is seen as not being able to do anything about such blatant violations of Chinese sovereignty.
While the U.S. does engage in media manipulation and utilize false narratives to stir chaos, at a fundamental level the problem for China is that a very sizeable percentage of HKers are and have been long time opponents of China. HK is a vulnerable target because of this and it was easy to fan the flames once the media could turn the narrative around the extraction bill.
The vast majority of these so called "pro-democracy protestors" aren't rationale, they are immature kids / young adults who are throwing away their futures for a cause that doesn't exist and for goals they won't achieve. Not to mention social and economic problems like high housing prices, high pressure in HK only made the general population in HK easier to manipulate.
IMO, China needs the HK government to remove those in power who sympathize with protestors and hand out harsher punishment for those arrested. It's comical that HK police has only arrested ~4,500 people in over 5 months (not to mention many of the arrested were let go with a slap on the wrist), compared to the U.S. police that arrested over 10,000 in just 5 days during the LA riots. The longer the riots in HK drag on, the more unpredictable it becomes and risks even higher escalation.
One has to wonder if China is waiting to see what happens with Trump and the possible threat of impeachment. It seems like China is reluctant to take action on both the HK issue and the trade deal until then.