Putting aside of what they plan on doing ... let's assume they don't ban foreign exchanges (yet, or ever). Is there any reason of why a Chinese citizen wouldn't be able to buy crypto's from a foreign exchange (USD/EUR/GBP...etc)? Again, I'm not really sure how Chinese banks work when purchasing anything online in a different currency, I assume it's no different to the UK where if I pay for something in USD, the bank will just convert it using their rates and charge a foreign transaction fee.
That's the thing isn't it? If China really wanted to prevent citizens from exchanging their crypto to fiat, they'd have to go after (block, rather) all means of accessing exchanges operating outside their borders. You're right of course and Chinese banks work the same way. You give a person anywhere in the world your bank account and you get it in the account's currency, regardless of what currency it was sent in. The receiving bank is the one who decides if they can accept the payment currency, and then it's all converted at interbank rates. That's actually a major part of banking income.
China would not want to prevent this, since this makes it even easier for them to raise alerts at bank level if receiving money from abroad.