No, the 15.3% is only the SE self-employment tax and does not include payment for estimated income tax. See the following citation in the section titled "What are My Self-Employed Tax Obligations?' on the IRS web page at:
https://www.irs.gov/businesses/small-businesses-self-employed/self-employed-individuals-tax-center"Self-employed individuals generally must pay self-employment tax (SE tax) as well as income tax. SE tax is a Social Security and Medicare tax primarily for individuals who work for themselves. It is similar to the Social Security and Medicare taxes withheld from the pay of most wage earners. In general, anytime the wording "self-employment tax" is used, it only refers to Social Security and Medicare taxes and not any other tax (like income tax)."
