Why would you tell that to them? Schadenfreude? It's just asshole behavior, and probably false even under your own definition (you should read up on medically defined biological sex, chromosomic vs genetic, gonadal vs apparent, also sex hormones and embryological development of primary sex characters), not to mention current definitions that are much more useful in practice. And even if you want to keep believing that trans people are just delusional and their ideas are completely false, think whether it would be fine for an atheist to laugh at someone whose only reason to keep going is God.