It depends on what one means by truly bilingual. In the context of this article, I took it to mean "no accent" and able to speak in one language as comfortably as the other.
Bilingual people are a dime a dozen, but it's fair to say truly bilingual people are pretty rare, and almost invariably learned both languages at a young age.
I'd argue that it again depends on the region - as GP said in some places it's very common to speak multiple languages.
I'm English and live in Lithuania now, and many of my Lithuanian friends can speak perfectly good English without an 'accent'. It's also not uncommon to speak a third language like Russian or Polish, especially if the family has their roots in that country.
These languages do have some shared sounds, but are definatly not just dialects of each other.
A bit OT but the idea of no accent is kind of funny. My kid goes to an international school in Thailand. I hear my kid's classmates speak English with Australian, British, Canadian and northeastern American accents plus an accent we call "international school accent". I'm from California and of course I think I don't have an accent. Same as those kids and their parents. Yet I have a hard time understanding some of them.
Occasionally I manage to do a "Henry Higgins" and detect a main accent and sub-accent; there are a lot of hints in dialect though, like what you call a bread roll ...
Bilingual people are a dime a dozen, but it's fair to say truly bilingual people are pretty rare, and almost invariably learned both languages at a young age.