Let me do the math here. At the time this article was written Steem was diluted by 100% per year, but 90% of the dilution was paid to the SP holders, so the effective "interest rate" on SP was -10% per annum! Note the negative sign!

In December the dilution rate was changed by community vote to 9.5% per year. That reduced the effective "interest rate" to only -0.95% per annum.

Steem does not offer compound interest in any sense of the word. If you are not active on the platform, your holdings are diluted by about 1% per year.