You confused “obscurity” as in a synonym for low popularity with the word “obscurity” as in people not knowing how it works and people deliberately hiding the inner workings of a system.

Everyone using Linux can know how it works, that’s the opposite of obscurity in the sense it is used within “security by obscurity”.

Apart from that, Linux is very popular, just not on the Desktop. It is therefore not obscure in the sense of popularity either, at least the components which are hit by the bug mentioned in the article.