The market has not overreacted, in my opinion it is a fair reaction, since in China every policy is long-termed, it wont arise overnight and it wont disappear overnight, the trend will be there for years if not decades.
So once a policy-change occurs, you need to price that into your asset.
The anti-trust regulation suggesting China is about to do something to capital giant, suggesting in China capital is still ruled by politicians, not the other way around like the case in the US.
As for the contribution of AliBaba, sure it is pioneered many tech in China, but it also kill many small business owners in the process and innovations, and the internet Giants make new tech start-up harder to rise due to the tech and capital advantages they enjoyed, coupling with the rather poorly enforced protection of intellectual property rights, for anything you invented and made, they can do better for half the price with probably more mature tweaks and better advisement just a few months later.
So these internet giant today have too much role in China, they are now try to control the social media and control the public views and Jack Ma go even further: want to control the financial muscle of the country.
They are now way over-their head, Jack Ma once openly declared "working overtime (for the capital owners) is a bless", openly challenging China's labor rule in a in-your-face style. And due to the fact many social media were bought/controlled by his Alibaba group, that comment were not draw many criticism.
So the anti-trust law is much-needed and will be long-termed, China should not fall in the hand of large captial groups like the case in the US, giant captial groups are not too big to fail, they are just too big to make any positive contribution to the betterment of country than almost all alternatives.
As for rising capitals from oversea market, well, to be honest, capitals rised in oversea market play a very insignificant role to China's economy, for a few invidiual company maybe, but thats wont be a factor to concern for policy makers.