I would guess that they were born here, or came here as a very young child. You'd need to have fairly good mastery of English to correctly interpret those questions.
I learned a foreign language to fluency at age 17, and math is one of the easiest things to read in the second language because the grammar is never very creative and the vocabulary is much more restricted than a novel or even a typical newspaper.
(Tangentially, a lot of people born in raised in India speak English at near-native levels. Especially the well-educated classes that would be likely to move to the US and participate in something like this.)
English really isn't that hard to learn.
"At least 5000 people bought my books..."
Don't ask me why, but to me it should read instead:
"at least 5000 people have bought my books,"
or even:
"over 5000 people have bought my books."
Indeed they did, until very recently[0]. I had no idea.
[0] http://www.quickanddirtytips.com/education/grammar/more-than...
Also, what you're describing goes beyond native level English and reaches the level of professional writer/teacher/linguist/editor/etc level English. I would expect many HN readers to have English proficiency beyond native level.
As for errors, I've noticed that spending time in the US and using colloquial English a lot, has wreaked havoc on my English grammar. Or maybe it's the distance from schooling that did it.