It's fairly basic qualifications. It's not difficult to reach millions of reads over 10 years, that should take less than a hundred questions or articles. Possibly a lot less back then when commercial content was less prevalent and Facebook/Quora/StackOverflow were in their infancy.

If you have a stackoverflow account with at least 500 points, click your user profile and check out the "people reached" metric in the top right corner. Prepare to be amazed.