is it that Herge and the Studio Ghibli animators were influenced by the same art style?
I think that you have more or less answered your own question - aspects of the Chinese and Japanese artistic worlds have influenced each other for centuries; there would be no real reason to say that Hergé was a major influence, although of course it is not impossible that the likes of Hayao Miyazaki know of and even respect Hergé as a peer.
Hergé was taught brush skills and (perhaps more importantly) how to apply himself seriously to art - philosophically, as it were - by T'Chang; that every mark made should serve a purpose, and be the best line he could possibly make it.
From Japanese art, specifically the Ukiyo-e
("Floating World") genre, Hergé took the notion of parsimony, that the simplest design, the most efficient way of expressing an idea, was the best.
I think that Ebert is being a little presumptuous in stating that Hergé was a "disciple" of Hiroshige - the most commonly drawn parallels are between the way Hergé draws water, and the famous image of the Great Wave off Kanagawa
, from the series, Thirty-six Views of Mount Fuji
, by Katsushika Hokusai.
Hokusai was influenced by the earlier master, Sesshū Tōyō, who himself travelled to China to learn from the artists there, before returning to Japan where he was the founder of a whole new school of painting.
So on that level, yes, I would imagine that there are shared aspects and influences to be found in the outputs of Hergé and Studio Ghibli, but I think that they are both offspring of a common ancestor, rather than bound together closely (for one thing, the Studio Ghibli pictures I have seen maintain a level of detail and complexity in the images, unlike Hergé's pursuit of spare simplicity).