So the thing is that if someone makes a racist/cissexist/sexist etc point, and you respond with “of course a white person/cis person/guy/etc would say that” where you’re making a guess as to who/what they are: if you guess wrong, you’re giving them a twofold cover for their stance. It’s twofold because a) you are arguing that their stance is incorrect because of what they are, something they could easily disprove, and b) because you are arguing that if they were this other thing, which they are, then their stance would hold more weight, so it does.
What you are has to do with the probability that you’ve had some portion of a common set of experiences, and there are a number of interpretations out there for varying portions of that set of experiences. If you are marginalized in some way you are more likely to have had a large enough number of experiences to be compelled by the argument that that marginalization is systemic and is, well, marginalizing. It doesn’t guarantee that you will be compelled by that argument. It definitely doesn’t guarantee that you will be compelled by arguments about the validity of a marginalization you don’t experience. This is how there are people of color who believe racism is over, women who believe that feminism is bad for them, and so on.
What if you actually know what a person is, because they established it already? Functionally you’re still reducing their interpretation down to what they are, and telling them that what they are matters more than what they think. Yeah, you probably mean that what they are has more of an influence on what they think than they know, but that’s not what they are going to hear. If you’re participating in a discussion in order to check or change their opinion, or someone’s opinion, you’re probably not going to get far by telling them that what they are makes them do/think wrong things. (If you’re trying to rile up, or drive some people out, or whatever, you may have better luck, and there are valid reasons for those things, but that’s beyond the scope of this post.) You’re probably going to have better success by exploring how what they are shapes what they think.
A cool thing is that you can approach this pretty much the same way as if you weren’t sure what they were and where they were coming from: “Why do you think that? Let’s say your perception is correct, and these other people’s perception is also correct. Is there some way that all of these perceptions can be true? What might cause you to have a different perspective than they have?” You might be able to get someone thinking a little more subjectively. There’s even a possibility they’ll start thinking a little more subjectively right there in the middle of the argument (this is extremely unlikely; it’s much more likely that they’ll have an ah hah moment in the shower the next day that starts a long slow process spanning months if not years). But again, this is all about probabilities. There are no guarantees.