The term "refutes" seems incorrect here. Perhaps "denies" instead. I was expecting a public demonstration of proof or evidence against the claim beyond an assertion.
how do you prove a negative?, all you can say is that within the bounds of the technology and skills they have to hand, they where unable to detect any intrusion, that does not mean that there is a guarantee that there was none.
The risk that there was may be very low, but its still there.
The only secure system is an air-gaped one, and even then there are evolving threats that circumvent that protection.
It's not a negative, the assertion was the the hack was carried out by a different country, namely China. This is a claim that could be backed by evidence but was not.
