I always wonder what the definition of 'style' is that people who make this claim use.
A 'style' that changes completely depending on a combination of artist, demographic target, genre, and era (as the artistic and storytelling style of manga does) isn't much of a style at all, by my definition.
For a particularly extreme example, compare Fist of the North Star (80s shonen fighting series, drawn by Taro Hara) to Doraemon (kids' series, starting in the 60s, drawn by Fujiko F. Fujio). To get a little less extreme, compare either of them to The Rose of Versailles (70s shojo romance/historical by Riyoko Ikeda).