Nonsense. Your definition only requires native-like facility, not native-like learning conditions. In other words, the (many!) adults who learn second, third, etc languages well enough to have native-like ease in speaking the language (often this is perceived as "thinking" in the language, not needing to translate from another language into the target language, etc) are truly bilingual.

Native speakers are simply the benchmark for determining how well a language has been learnt. There are plenty of bilingual people who learnt languages as adults and are by any scholarly definition bilingual.