If you read the letter Match.com sent to him, you will see that he is misrepresenting it here. All they ask him for is proof that the claims he is making are true.
From Match.com: > We demand that you immediately cease and desist from making these false claims. If your position is that these claims are substantiated, please promptly provide me with substantiation for each of these claims...If disclosing the substantiation data concerns you, Match.com is open to entering into a confidentiality agreement.
I don't know why Match.com would be the people to follow up on a possible false advertising claim like this. I assume they could sue plentyoffish if they don't provide Match with evidence? As plentyoffish points out, it is likely that Match.com (also?) makes false claims, so this could definitely be bluster. But they definitely don't ask for plentyoffish to reveal their processes.