Your statement is not exactly correct; a company does not need to be certified to ISO 9001:2015 before becoming certified to AS9100 Rev D, you can just be certified to AS9100 Rev D without going through the first step.
The AS9100 Rev D standard actually includes all of the ISO 9001:2015 requirements, plus additional aerospace specific requirements. So, by being compliant to AS9100 Rev D you are also entirely certified to ISO 9001:2015. In fact, some certification bodies include both standards on the certificate so as to make it clear that the requirements of both are satisfied. This being said, you do not need to have certification audits for both ISO 9001 and AS9100, the one certification audit is sufficient, and by being compliant to AS9100 you are also compliant to ISO 9001; separate certificates and audits are not required.
For a better understanding of everything included in AS9100 Re v D, see the whitepaper: Clause-by-clause explanation of AS9100 Rev D, https://info.advisera.com/9100academy/free-download/clause-by-clause-explanation-of-as9100-rev-d