Sorry if I seem a bit disputatious.
I don't know Japanese, are you drawing that conclusion from the original Japanese? I'll take your word for it if you are.
I checked the missdream.org scanlation, and it says that she has 1 child a millennium, for her whole life, which would allow her to have a son in the first Silver Millennium. And in the Kodansha US release, she says "I've heard that for generations, Millennium's queen has borne only one princess!" which doesn't seem to exclude her having a son.
The first would, however, preclude Hyperion being Usagi's brother. Maybe he could be her father?