It's not enough to get rid of things they don't want you exposed to, it's that societal norms for what is considered normative and taboo have to be strictly controlled.
Some mention (as a commenter below) the fact 'Anyone can access outside withe a VPN! - but to some extent this is not the issue.
It's probably just fine if a small portion of educated people can access Wikipedia. What matters is hard control of the critical mass of citizens.
Admittedly, China would have to shut down specific access in the case of Twitter mobs etc. - they won't allow that 'easily with a VPN' etc..
If the average Chinese remains 'pragmatically loyal to the state' then it's a win.
I think the most powerful part of that strategy is to infuse and conflate the CCP with 'China' itself and with ethnic Han Superiority itself as a defacto subtext.
You can really influence people a lot if you can control most of what they see daily. If the 'Daily Propaganda' is good, i.e. doesn't look totally hypocritical even in the face of facts, then it will work.
Of course it happens in America and other places to greater and lesser extent, but China has taken it to the next level.
I fear that Cuba will not only take China's cue, but literally invite them in to help - and that this will spread.