1. 中华人民共和国 has laws.
2. 中华人民共和国 is well connected to the internet.
3. 中华人民共和国 can project its interests
around the world easily in rather nasty ways.
4. 中华人民共和国 can project its interests from
within its borders.
5. 中华人民共和国 has an interest in controlling
commerce within its borders.
I believe this is an act of foreign policy, not domestic. It's not about unplugging citizens from the internet. It is about achieving some parity with other state level actors in regard to what is and isn't allowed on the internet.中华人民共和国 's interests are orthogonal to those of the US and UK. It is not so much interested in the internet as an organ of a surveillance state or as an alternative source of foreign intelligence in lieu of boots on the ground.
The mechanics of the attack are entirely within the realm of sanctioned internet behavior: visiting a site places javascript in the browser without explicit approval of the end user. The javascript may do something not in the user's interest. The javascript may generate unnecessary internet traffic. The purposes for which the javascript does so are solely the purposes of the site injecting it.
The great wall comes with terms and conditions.