I believe I already explained how I got to that number in the article. I already have 400 articles, with 50 earning which results in 3-figures. Adding one more zero to both would suggest that 500 articles will give you 4-figures. But if your posts are of good quality and well-niched, 400 earning articles will get you to 4-figures.
However, not all of your articles will be earning articles. So, it is safe to assume about 40% won’t earn. This is the additional 300. So, altogether it makes 700