She's not saying that black people can't make racist statements and that they get a completely free pass when it comes to being racists. She's not. Stop saying that she is.
She's defining racism with a particularly sociological bent. She's saying that racism is an act by an oppressor towards the oppressed. When an oppressed person insults an oppressor they are operating within a power structure that has them at a disadvantage.
When she says "black people can't be racist towards white people" she is saying "the oppressed cannot, themselves, oppress their oppressors". That's why they're called "the oppressed".
Regardless of what you or I may say about her character, dragging this tweet out as evidence of her hypocrisy smacks of willful ignorance.