From the Business Insider article you posted:
> Walensky was referring to a new CDC study of nearly 4,000 front-line workers, some vaccinated and some not, who tested themselves weekly for COVID-19 infections between December and March.
> But more data is still necessary to say so definitively, which is why researchers are recruiting thousands of college students across the country to find out more about the likelihood of asymptomatic spread of this virus among vaccinated people.
The article is explicit that the CDC was talking about the findings in a new study, and that more research still needed to be conducted before being sure of their findings.
> It was not until July and the CDC MMWR report on the Providence, Rhode Island, bear week festivities that lead to unexpected breakthrough infections that the CDC admitted the vaccines were not 90% effective against infection anymore.
Because of the new delta variant that those very same festivity goers helped the CDC with[1], and that changed the CDC's guidelines:
> The speed of the investigation — and the exceptional participation from the mostly gay men involved in the outbreak — helped the CDC learn new information about the delta variant. And it was that new information, in part, that prompted the agency to change its guidance for how vaccinated people should keep themselves safe at this stage of the pandemic — including a return to masking indoors.
[1] https://www.npr.org/sections/health-shots/2021/08/06/1025553...