Ethan, I think you are conflating my suspicion about how readers do read with a proposal about how readers should read.
You know that Dilbert cartoon where step three is "PROFIT!" and step two is missing? There is a common tendency in discussions about racism to perform a similar trick of prestidigitation, where step three is a colorblind society and step two is missing. It's sort of like a fear of sympathetic magic: if I assert aloud that readers read unmarked characters as white, it either makes it more likely that they will, or is as if I am suggesting that ought to.
But actually the opposite is the case. I think it would be great if readers did not read unmarked characters as white. And in some cases they may not. But it is an empirical observation that they in fact do.
I said nothing about any John Baker. I mentioned Hari Seldon. That name does not sound particularly white. "Hari" is in fact one of the names of Vishnu, and I bet that if you were at work in an IT company and they said in the daily stand-up meeting "a new employee named Hari Seldon will be joining us Wednesday" your mind would not naturally flash to white guy. So it is notable, as an empirical fact about literature, that when I read Foundation as a kid I imagined Hari Seldon as white, and I will wager so did you (and if you "didn't imagine his race", again, imagine someone had asked you to draw the scene), and I will bet you five dollars, if she's read Foundation, so did Janet: ask her.
Similarly, "Susan Calvin" is a completely plausible African-American woman's name (and John Baker actually is a similarly plausible African-American man's name; most African-Americans descended from slaves in fact have English surnames, so a Jewish or Polish or Dutch surname would be a much clearer marker of whiteness in America than "Calvin" or "Baker"!).
You can tell by introspection that you, as a white reader, do this. It is an interesting hypothesis that perhaps people of other races imagine unmarked characters (for instance those with wholly invented names of no known ethnicity, such as "Zaphod Beeblebrox") as their own color. It is not, however, empirically the case, at least from those people of color -- raised in the U.S. -- I have read or spoken with on the topic. Mary Anne Mohanraj does not. Samuel Delany does not. Tempest Bradford does not. Octavia Butler did not. In fact the common experience reported, at least back in the day when most characters were relatively unmarked, is of bursting into tears upon finally encountering a character specifically marked as nonwhite. That sure does not sound a whole hell of a lot like they were complacently imagining that all the characters were, e.g., black.
If is profoundly problematic to define white as normal. It is in fact the root of tremendous iniquity and evil. It is shooting the messenger, however, to accuse someone pointing out that that is how we have been indoctrinated, of doing the "defining". The defining is already done -- I am trying to resist it. But seeing it is a necessary step in resisting it.
Now, specifying white characters as white is an interesting proposal, and indeed Mary Anne (whose article on this you should really read) notes that you should go beyond that, and in fact mark your white characters as what they are -- half-German half-Lithuanian, or of poor Scotch-Irish hog farming stock from the Tennessee river valley, or second generation Greeks who went from a fishing village to the Bronx, or whatever. Because the assumption of generic whiteness also erases white ethnicity.
And when Mary Anne was preparing the essay which you really should read, and asked for my feedback in email, one of the few things that I quibbled with was the word "mention" in the suggestion that you should "mention" the character's race or ethnicity, because that's often, as a matter of writing technique, clunky. She revised the relevant paragraph to read:
Give your white characters an ethnic and cultural history, even if it ends up barely mentioned in your story. But be sure you do indicate it somehow — it’s not enough for you, the author, to know their history. Unless the reader gets an intimation of it as well, that character might as well be generic white. Names, clothes, hair, foods, diction, holidays, memories, can all signal ethnicity to the reader without a blinking neon sign over their heads saying “fifth-generation Irish-Croat”.
You don't need to state ethnicity to break the empirically observed phenomenon of default whiteness -- but it does need to be on the page somehow, because the page is actually the only medium of communication between the author and the reader. Perhaps this is what you mean -- that if the author imagines the character's race, without ever intentionally signalling it, subtle signals will creep in that will influence the reader to imagine properly. Sure, fine. That's a suggestion of a writing technique. The fact remains though that something -- even if it's some subtlety of diction or dress the author is not fully conscious of -- must have gotten to the page.
To sum up: I am not proposing that authors who wish to have characters read as white comfortably refrain from giving any signal of race. Rather, I am warning that that is how unmarked characters get read. If I was concerned that my stories had perhaps too few white characters in them, then I might have counted differently!