The distinction 'kragen is making is between actual cases (total number of people with the virus) versus cases confirmed via testing. The daily growth of ~33% probably reflects increase in testing capacity more than increase in actual cases.

Here are some studies supporting the doubling time estimate of 6-8 days:

https://wwwnc.cdc.gov/eid/article/26/5/20-0146_article

https://www.nejm.org/doi/full/10.1056/NEJMoa2001316

https://www.thelancet.com/journals/lancet/article/PIIS0140-6...