I discussed the high writing count with the woman I linked to above. She told me that she writes for herself only, has no intention of ever publishing anything, does it for fun, and never edits.
As for whether or not the high word counts are possible, as far as I'm concerned, absolutely. Doing word wars, last year, I was consistently getting 1500+ words per half hour, or 3000+ words per hour. To reach 500,000 words in 30 days, the math boils down to 16,667 words per day. My top writing day was 16,250 and I wrote for less than 8 hours. Difficult? Brain bleeding? Yes. Possible? Also yes.
You know, NaNoWriMo is not just for writers seeking publication. It's for any kind of writer, regardless of their end goal.