The implication is invalid if you accept the correct meaning of “portrait” (“a depiction expressing character, most often focused on the face”), because not all busts are portraits (and fwiw, not all portraits are busts).
Even if you accept the over-restrictive definition of “portrait” used by the site, (”[a]n image primarily of the face of a character, (*)”), then the implication is still invalid. “[B]ust” is defined on the site as “[a]n image of the upper body of the character, no lower than the navel” (mostly correct). By the site’s own incorrect description, a portrait can never have anything below the shoulders visible in the frame, and this includes the area between shoulders and navel.
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