No, not all start as light novels. In many cases the light novels come afterwards as a way to capitalise (eg Demon Slayer, the manga finished a while ago, so while the anime is still running light novels are coming out).

Apothecary Diaries started as a light novel, but JJK, AoT and many others start as Manga

Also, the article is about animes as well. Makato Shinkai’s She and Her Cat is an anime, not a book, for example. It talks a lot about books, but it isn't only about books. I thought that was obvious.

> No, not all start as light novels

Many are though, many of the animes that came out for a few years I had already read the LN for. That so many novels becomes animes is likely a big reason why there are so many novels being written along those styles.