I think you're right that people who talk about inclusivity on Twitter don't necessarily show up to buy indie books.
But I don't think there are many people policing who can write what (and by policing I mean complaining online. They aren't really doing more than that). I have never once seen a criticism or review comment suggesting I was not the person to write something wheras I have had a number of comments that celebrated the inclusivity of a book.
There are apparently enough to get a starred Kirkus review taken down and revised.
https://slate.com/culture/2017/10/kirkus-withdraws-starred-review-after-criticism.html Granted, that was in 2017, but I'm not sure things have gotten better since then.
I can understand objections to portrayals of a group that are stereotypic, otherwise disrespectful, or just inaccurate. But in the case cited above, the publisher bent over backward to be sensitive and accurate. The issue was in part a) that a white person was the main character, and b) that it took the white person too long to awaken to injustice (which seems kind of realistic to me--people often do take too long to awaken to injustice). However, if the main character had been an Iranian Muslim instead, the novel might well have been criticized because a white author was using an Iranian Muslim main character. I forget the title, but that's exactly what happened when a white author wrote from the viewpoint of a black, inner city teenager.
I don't know how many people take this view, but it effectively puts white authors in a box. If your main character is white, regardless of what the character is like, you're guilty of writing a white savior narrative. If the main character is not, you're guilty of appropriation or simply of telling a story that isn't yours to tell. If all the characters are white, you're guilty of not being inclusive enough.
The one advantage to being a prawn is that I'm too far out of sight for the mobs to take notice of me. Even so, I do my best to be inclusive and to represent other groups fairly, to the best of my ability. Although my MC is often white, normally the heroism in the group is a group effort, not one person saving the day. I do also have one book with a black MC, one book with a Brazilian MC (mixed black and Portuguese ancestry) and three with MCs of Asian ancestry. A number of the other books don't really have one MC but have an ensemble structure.
In the interest of accurately portraying people, I try to stick to characters that have some resemblance to people I know. For example, the black characters draw on students and their parents, whom I met through teaching. Since I was teaching at Beverly Hills High School, it's not a representative cross-section, but at least I accurately reflect real people to the best of my ability.
There's always room to improve, but when you look at the other various groups were portrayed fifty years ago, at least we are making progress.
