It is generally considered a Ghibli movie because the producers and director formed studio Ghibli on the back of the success of this movie. According the other comments, they even hired animators from the animation studio.

So in a sense, Studio Ghibli is the product of Nausicaa, and Nausicaa is the product of the not-yet formed Studio Ghibli.

So kind of like how Daicon III/IV lead to Studio Gainax then

https://en.wikipedia.org/wiki/Daicon_III_and_IV_Opening_Anim...

https://www.youtube.com/watch?v=yDN68LxWW-8

I discovered Nausicaa on Netflix last year, and was so impressed! Incredible world building, and you can see so much of its DNA in other media which has come after. Rey from the new Star Wars for example seems almost directly lifted from the film.

I recommend reading the manga (written and drawn by Miyazaki) on which the movie is based, which is on another level, and has a nice English edition from Viz: https://www.viz.com/read/manga/nausicaa-of-the-valley-of-the...

Iirc, the manga was started shortly before or even during actual production of the movie, but completed and published much later. So it's not a straightforward "based on", in many ways - it's a bit like the latter part of Game of Thrones. But I agree that the manga is better, simply because it has time to fleshen out every aspect of the story.

It was designed by Miyazaki though, who also wrote and drew the manga (which is better than the movie, having space for more material, of course). You can see Miyazaki aesthetic throughout all his works, pre and with Ghibli.