When they're talking to cisgender people, they claim that gender is performative (e.g. created by actions). "You're not a real X because you do Y / don't do Z."
When they're talking to transgender people, they claim that gender is generative (e.g. created by genes). "You can't be A because not-B."
In order for either of those arguments to be valid, it would have to apply to everyone. If gender is performative, then all people (regardless of genes) who perform the set of actions socially designated as "feminine" must necessarily be women. Others are not, or open to challenge. If gender is generative, then all people (regardless of actions) with a Y chromosome must necessarily be men. Others are not, or open to challenge. Instead, there is no unmarked case and nobody is ever really safe. The existential insecurity is created to threaten people, not because it has any actual consistency. If it were all one way or all the other, they might be bigots but they wouldn't be hypocrites.
But that's not how gender police work. They always argue whatever will get them what they want in the moment, regardless of whether it contradicts something else they just said. They're like the chaotic neutral party member who makes selfish choices and gets the whole party killed. When someone contradicts their own arguments, you know that none of it is valid, it's just a bunch of bullshit propaganda, and you should ignore all of it.
Furthermore, the very fact that gender police use both (invalid) arguments means that they have disproven another of their own fundamental premises: that gender is in some way absolute. If they frame that immutability in two mutually exclusive arguments, then neither is actually dominant and gender is instead a social construct -- which makes it mutable, because anything constructed by people can be changed by people.
In order to damage an attacker's reality tunnel, simply cross the streams. Take whichever argument they start with, speak as if it is true, and give examples farther and farther from their tolerance until they break and switch to the opposite argument. Then reverse and repeat the process. After you have run them over and backed up to do so again, highlight the contradiction and declare that they have disproven their own assertion of immutability by making two mutually exclusive arguments. Sometimes this will merely alert bystanders to the bullshit -- but reality tunnels built on sand are inherently unstable, so you stand a significant chance of doing permanent damage that will impair the attacker's ability to hurt people in the future. Do it all with words of logic, and it's perfectly legal.