So I'm trying to understand that you're coming from a place of not understanding the trans population as much or what it means at a medical and psychological level to be transgendered, and not jump to conclusions that you're just trying to oversimplify this so it takes as little effort as possible to wrap your head around transgenderism (just like I didn't know as much as I do now before I was involved in various advocacy groups and took and then taught cultural awareness about the LGBTQ+ population- education is a journey and I'm assuming you're coming from a place of not understanding, instead of callous dislike or disinterest). And to be fair, it is a difficult concept to look at both transgender and "transracial" topics. Gender and race are social constructs, yes, but not to an equal degree. Gender is more deeply rooted in your own mind, while race is more forcibly imposed by our society.
So being trans is something you know from a very young age and usually you come out when you're mature enough to understand why you've felt different your whole life and have the terms for what you're feeling, so it's not something you eventually decide upon and leverage to your advantage. And that's assuming the best timeline as many trans individuals for various reasons feel they can't come out for a lot longer into their lives. Even the transition surgeries are now called Gender Confirmation Surgery. Because trans individuals feel that the lie is the biological sex they were born into, that they have never identified with. This whole process (and it's a hell of a process because it's not as simple as scheduling a surgery- there are several psychological evaluations that have to happen, hormone therapy for a set amount of time before you are able to sign up for surgery and then the legal process of changing your gender on documents, etc.) is traumatic, intense, scary (discrimination and violence against trans individuals happens at alarming rates), and many trans teens and adults never make it through because while the gay youth population is 4 times as likely to attempt suicide, trans teens are twice that- so 8 times as likely to attempt (and often succeed) at taking their own lives (with similar rates in trans adults). This isn't about lying because a trans person isn't trying to get anything out of other people by transitioning, they are merely trying to live their truth and embody the gender they have always identified with. This is a trans individual, and cis-gendered individuals (like me, for example) can't claim to be trans even if we feel a strong kinship with the trans community but can only be allies.
Now onto Rachel. The reason there is such outrage is because she's playing both sides- she is benefitting from acknowledging her actual whiteness (sueing Howard University for discrimination for example) as well as allowing herself to indulge in blackness as a commodity, without having to actually engage with every facet of what being black and growing up black entails, namely discrimination, marginalization, oppression, etc. And while that's one part of the lie that has people outraged, she has also used her deception to leverage personal benefit like applying for a strictly black-only scholarship, to network as a black lady to advance her career, she had claimed to be a victim of hate speech towards black individuals, etc. And she HAS to at some level conscious of the statements and actions she is engaged in and that she had a white experience and has not suffered what those in the community around her have. To know that and still give speeches where she claimes to have gone through the same experiences as a proud black woman to a community she is supposedly advocating for is what is really upsetting a lot of black individuals. She is lying to the community she is claiming to want to help and advance forward. You can't have true advancement with that level of dishonesty. And also, she's adding another level to the alienation between the black and white community by doing this, which undermines her goals even further.
In the end it's about honestly (Caitlyn came out about who she honestly is) and the harm that Rachel's lies have brough towards the black community, both as a community leader and her commoditization of struggles she hasn't had to deal with growing up.