That's not believed--OneShotHelpful is wrong. Read their Wikipedia link carefully--there's no support for the claim. See also https://en.wikipedia.org/wiki/Ornithischia

>Ornithischia (/ˌɔːrnəˈθɪski.ə/) is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds.

The key word here being "superficially".

>However, birds are only distantly related to this group as birds are theropod dinosaurs.[3] Ornithischians with well known anatomical adaptations include the ceratopsians or "horn-faced" dinosaurs (e.g. Triceratops), the pachycephalosaurs or "thick-headed" dinosaurs, the armored dinosaurs (Thyreophora) such as stegosaurs and ankylosaurs, and the ornithopods.

Only the last of those was bipedal. And of course all of these bipeds had quadrupedal ancestors--we're all tetrapods.