At the risk of being ungenerous, I am going to admit that it doesn't strike me as at all certain that anyone did divine that you were white. Someone made an assumption that happened to be at least semi-correct, but that doesn't mean it wasn't a pure accident that her assumption was right. One of the patterns I've seen over and over again in this past year is that very assumption that a speaker is white, followed by the revelation that no, the person being told she sounds white is very much not.

And it's precisely this sort of thing that makes me cheer for the masks the Internet allows us to wear. Yes, there are situations where it's useful and important to be able to tell people in a conversation where you're coming from -- but for every situation like that I think there's probably one where not knowing, and the very experience of making wrong assumptions and having them corrected, is enlightening and invaluable. It's one thing to hear "[Group A] is not a monolith," and quite another to run up against undeniable evidence that no, others in your position do not always share your opinions, and may have good reasons why they do not.

As you say, it does rather overstate matters for me to assert that pseudonymity forces people to focus their thoughts on the substance of what participants say. There are many people who'll do their best to drag a discussion back to questions of who the speaker is, rather than the content of the speech, no matter how pseudonymous everyone is. But pseudonymity at least makes that difficult, and as a structural matter makes it almost impossible for them to take over a conversation in the face of other participants' resistance.