Yes, you attempt to argue about hypotheticals. In practice fighting disinformation is about fact checks by various parties, including - but not limited to - OSINT researchers, scientists, independent journalists, news orgs, NGOs and government agencies.

Can some of these be corrupt? Absolutely!
Does it matter? Only if the piece of news in question cannot be corroborated by other parties.

The basis of news has to be facts that can be checked in order to be confirmed or debunked by other parties. There is no possibility to determine with 100% certainty if a piece of information is correct or not, because there is no objective observer who could make that kind of judgement. All we have - and always had - is simply the possibility to check and compare information with the underlying facts.

It is a messy process, but that does not mean that we cannot reach a consensus on which pieces of information are not supported by fact and that this is misinformation. Such consensus is part of the social contract.

Ultimately the consensus will never be supported by every involved party, but that does not mean that consenting parties aren’t allowed to take appropriate action against the misinformation.