Pfizer didn't. Their press release distinguishes between SARS-CoV-2 and COVID-19, only claims they prevented symptoms in people not previously infected by the virus, and didn't make any claims about infection/transmission. It was the conflation of the two terms in the media, referring to the virus as "COVID-19", that resulted in this mistake.

And even so, for several months articles were coming out questioning of it stopped infection/transmission at all. It was only once we got a few months into 2021 that that got buried with the assumption it did.