Wiktionary and thefreedictionary.com express that this phrase refers to the subjective belief of a person telling a joke (and I believe that extends to someone laughing at one) rather than that every joke expresses some objective truth.
However, this brings up the idea that subjective biases can prevent someone from actually understanding reality, so independently declaring that something is "100% false" is still dubious.
Do link-aggregation platforms like Lemmy have a documented method for marking things as true or false, other than by trying to change the relative number of upvotes? Community standards regarding what content should be most prominently displayed are documented for Wiktionary and Wikipedia.
Regardless, I don't want to discuss the implications of an aphorism on racism, as any disagreement in that discussion may be interpreted as being against the rules of our instances.