Why is it called "strong media"?

It's just an idiomatic term, so it's not the same as a specific academic concept (just as "mass" is not the same as "mass" in physics).

What you call "strong" in academic terms is a noun, but what is usually said becomes an adjective.

It's sort of a blurring of the line between academic and everyday language, but it depends on what people accept.

So it's okay for us to call a strong media outlet a strong media outlet according to a publicly accepted and "conventional" understanding, but if we're talking about it in an academic context, we should still use it according to academic concepts, and avoid using this kind of inaccurate conceptualization of the term.