If the Steem money supply goes up by 9% a year and the SP holders get 15%, that means if you hold Steem power and do nothing instead of getting 9% you will 9 * 0.15 %. In other words 1.35%.

The article says

"if you hold 1000 SP [ and do nothing ] you will have 1090 SP at the end of the year."

This is wrong. Now, if you are posting and getting curation rewards and always choose 100% power up you could indeed have 1090 SP by the end of the year. If you do nothing you should expect 1013.5 Steem Power by the end of the year.

If these authors of this news letter are unable to understand the way things are now, how are they supposed to be able to predict the future?

Crypto-currency dropped down suddenly by 20%. I blame Christmas. People are liquidating longer term investments to buy things for people they don't need or want, in order to carry on a tradition.

Money is for buying things after all. I don't think people are reckless for using their own money as they see fit. Bu the sudden drop in the crypto-currency market cap makes me reflect and believe that many were overly invested in bitcoin and alt-coins.