Re: your quote above, you are right that when it comes to issues of racism etc, white readers will "relate" more to white characters, at least in terms of them not having to endure racism and so on. Do you feel that PoC readers can relate to white characters, if the reader has endured racism but the white character never has, and is just going through life oblivious to that sort of thing? This is me just focusing on the racism aspect too, not other aspects of general bigotry that I as a woman could relate to in a character I'm reading about, regardless of their colour.
PoC are expected to relate to white people from the time we are children. We are not given a choice. As a child, I never expected to read a book about someone with my background. It wasn't an option.
Also, racism has had very little effect on my life to be honest, at least in part because: 1) I live in a very diverse Canadian city, and 2) I am ambiguous-looking and have a "white" name...so some people might assume I am white. At this point, it has more effect on me in publishing than any other aspect of my life.
There is so much more to being PoC that has nothing to do with bigotry, and I don't write much about racism in my stories. I just wrote a book with a Chinese-Canadian hero and heroine, and racism was barely mentioned, except as it related to the hero's family history. (His family had been in Canada for over a 100 years and had to deal with head tax, exclusion act, etc.) But it was really a romantic comedy about mental illness.
Actually, I think part of the problem is that some white people expect books about PoC to be about oppression (and the PoC ultimately being saved from oppression by a "white savior"). They don't like to think about us having normal lives. Books about PoC are sometimes expected to be "misery porn," which is gross. On the other hand, pointing out micro-aggressions in day-to-day life might be uncomfortable for some white readers who may do such things themselves. Similarly, books that challenge stereotypes might also be uncomfortable for white readers who--perhaps subconsciously--believe those stereotypes. They don't want their world view challenged.
From my POV as one of the whitest white people you can get (e.g. Scottish/Irish heritage etc.), when it comes to reading books with PoC (or watching movies/docos that include those POVs) I may not be able to "relate to" racism experienced by the characters or people (except as a thinking, feeling, compassionate human being who cares about other human beings) because I haven't gone through what those people have. But I expect that I will be able to relate on some level, e.g. if it's a romance I can relate to the romantic woes, or if it's got family drama I should be able to relate on some level there. So I don't get why editors/publishers wouldn't see that as well (but I agree that evidently they don't). Just because a character is a PoC doesn't mean *any* reader shouldn't be able to relate to what they go through at least on some level (even if not the racism side of things).
Last night I watched a doco about transgender people in New York from the 60s and 70s through the 90s/2000s and what sort of stuff they went through and are still going through - I can't relate to them because I'm not trans but that didn't mean the story didn't make me get teary and angry about all that they've endured and how marginalised they are. But I absolutely agree that I would not be able to relate to what they have gone through.
The fact that people can't relate is a little baffling to me because works of fiction typically deal with basic human emotions. In many cases, I think people decide they can't relate beforehand and go into things with pre-conceived notions. I don't fully understand it.