As always, take these rumors with a grain of salt. It's possible that September is merely an announcement date, and not the actual product availability date. Furthermore, it's not unlikely that the coronavirus situation will further shake up plans.
There are some rumors that AMD was originally planning to launch Ryzen 4000 at Computex in June, but ComputerBase sources confirm that was never the case. The series was not expected sooner than July.Intel's Comet Lake-S desktop processors are expected to be announced on April 30, with product availability on May 27.
In fact, AMD Lisa Su already confirmed in an interview that Zen 3 desktop will launch the end of this year. She reiterated this claim during Analyst Day in March.