How does believing that you're the opposite sex despite the obvious evidence to the contrary not a delusion?

There's lots of aspects to a person's sex. There's genitalia, which can be indeterminate. But the brain also different between men and women. When that's also indeterminate, or opposite to the genitalia, then you have a problem that can be diagnosed and treated. If you treat it with gender affirmation, you get much better outcomes, and you usually alleviate other psychiatric disorders that might be present.

The entire brain argument falls apart when nobody can define what a female brain is.

We know what a female brain does. So the test is to ask the patient. You certainly have to do it over a period of time, and look for consistency.

If so then what about tomboys who never show any gender dysphoria?Are they trans in denial?

No.

Can you give an answer that doesn't resort to gender stereotypes?

Again, the diagnosis is established by talking to the patient.