I have never seen an accepted fact checking site answer this, which is very strange since it is such an enormous and grave conspiracy theory if it were true. The Mueller report is extensive and quite conclusive in stating that no such evidence of collusion (conspiracy) was not found. Yet fact checkers are happy to check peripheral and far less consequential claims around the case for some reason (e.g., https://www.snopes.com/fact-check/mueller-report-no-obstruct...), but are strangely hesitant to address the elephant in the room.
Or for another example, there were many false or poorly substantiated claims made about covid and vaccines during the pandemic. I saw "reputable" fact checkers address a certain set of those claims about the virus and drugs, but were strangely silent when it came to a different set of claims.
So fact checkers don't even need to provide false content at all, they can be very political and biased simply by carefully choosing exactly what "facts" or claims that they address.