Uh...well, then the site might be wrong?
See - The Magician's Nephew was started before The Lion, The Witch, And The Wardrobe, but not completed until long afterward. So yes, technically it was done afterwards, but the idea for it came first, and was conceived by C. S. Lewis as the beginning of the series from the very beginning.
At least, that's what I found out years ago when I was looking into the books.
And I have to say that I never thought the books were religious in content, even though I knew they were. That is, when I first read them as a child I had no idea there were religious overtones, then I found out as a late teen that there were, and I stopped re-reading them. However, when I recently came back to reading them again I didn't find that the content had very much overt religion - quite the opposite - so I wound up kicking myself for not having re-read them for all these years.
Makes sense?