I've thought a little more on the whole subject. What we certainly shouldn't forget is that there are people who are better than 99% of other ones -- by definition, on average, one of hundred around you is better in learning languages than all the rest. I have a friend who's such a talent: after learning the foreign language in school for some years and not leaving her country, she spent only three months in that foreign country and
sounds to the native speakers like somebody who was there much, much longer. Now if we'd ignore the years she spent learning language in her home country, it's easy to falsely claim that she did everything "in 3 months." Now the second question is -- what is that that she did.
If you would ask even professors to rate her based on what they hear, they'd give her the highest notes. How people sound is a very important point in the feeling other native speakers have, because those who are not top few percent have problem with that and for them, there is a good correlation between different aspects of their language capabilities. So if some professor is evaluating her for a C1 grade based only on whet he hears in a simple conversation with her, she'd pass it easily. However, if she'd have to write for a C1 exam and the same professor would have to give her a note only based on what she'd write, it would be another story.
Now consider also this: the author of the article we discuss is an opera singer. It's obvious that his "ear" is different than the one of an average person. He hears the nuances better. He is also preselected for his abilities to control his vocal tract enough to be probably one of at least one or even more thousands!
In short, I can easily imagine that he can sound even much better than an average guy who reached C1. Still, if C1 includes a written exam, it's not the same. Take a look at the message exchange between one talented person and me -- I believe him he's better than the most, but still it appears that he didn't understand the point and the arguments to which he replied even as they were in his native language. That's the really hard part, especially as I'm not a native English speaker and certainly not in possession of language superpowers, we can still see how hard is passing the gap between "good" and "the comparable to a good educated native."
That being said, if we compare, let's say, a million people, one of them will be by definition better than 999999 other ones. If the author of the article is one of those, it explains a lot, but then that's the major information missing in his article and the rest of it gives a false hope to 999999 of a million readers.
So I still believe that the article omits some important detail regarding that C1 feat of him. Most probably there is some mix of "small" details missing, just like in the conversation seen here -- some years of a foreign language in school, some years of learning and singing the whole opera pieces in French and a big predisposition for learning languages.