“On a title-by-title basis, of the 130,000 titles available on Kindle and in physical form, Kindle sales now make up over 12% of sales for those titles…".
So, it is 12% of those 130K titles, not 12% of all book sales on AMZN. I am not sure how many titles Amazon sells overall but it's probably much greater than 130K which means that the Kindle's share is lower.
Just wanted to point out that the post's title might not be entirely accurate.
(Update: it looks like Amazon stocks several million book titles)