Why is it a doujin???

Doujinshi just means it was self-published. So this is a manga the author published themselves without the help of any editor.

Researching the manga, and reading the bio on anilist, seems like this version is the original doujinshi that the author published.
They would eventually be hired by a French company to publish the most popular version of this manga officially, but because it was originally published in France, that Leviathan does not qualify to be added to the database (only comics originally published in Japan, Korea and China apply).

In the context of Japanese comics (manga), "Doujin" refers to self-published works created by amateur or independent artists. These works can include original stories, fan fiction, or artwork, and are often sold at events such as comic conventions or through specialized stores. The term "Doujin" is derived from the Japanese word "Doujinshi," which specifically refers to self-published manga or novels.

Source: https://www.quora.com/What-is-the-meaning-of-Doujin-in-Japanese-comics-manga

I'm betting your confusion is thinking Doujin = Adult, which is not true, it's just that most adult work is self-published, and the word sometimes is associated, incorrectly, with it.