This is one of the cyber attacks where the source is proven.

We all know who the attacker is here, but it's not literally "proven" to the standard of evidence that would be required in a US court. The attacker still has plausible deniability.

I know that this website is very USA-centric but I really fail to see what US courts have to do with the subject at hand. The question is more "as far as the international community is concerned, is there any reasonable technical doubt that the Chinese authorities are behind this?"

This is important for public discourse at least, because if it's technically undeniable that Chinese authorities are behind this attack then you can immediately assume than anybody saying that China has nothing to do with it is either acting in bad faith or is largely uninformed.

As we've seen multiple times in the past the existence or non-existence of conclusive proof is largely irrelevant when it comes to international policy anyway so the opinion of US courts is frankly besides the point.

“Someone else who has the ability to MITM millions of users in China did it” doesn’t sound particularly plausible to me.