It's not like that means much. "Seinen" just means it's published in a magazine which targets adults. That means they can cover more adult and complex themes than shonen would, but by no means that they have to, and manga can be moved between publishers (e.g. trigun and vinland saga were both originally published in shonen mags before moving to seinen, vinland was moved because its author didn't think he could keep up with a weekly publication).It's a seinen manga