Manga has some gore.
Readers: „iS tHaT sEiNeN?!“
Seinen is about the depth of the storytelling, as well as themes that are interesting to adults.
This manga has none of that (so far). It feels and is 100% Shounen. Weakling wants to rescue friend who protected him in the past. Can‘t get more Shounen than that.
I am not thrilled by the first chapter. Zero emotional involvement with the characters. Why would I care about their fate? Artwork won‘t stay that detailed either once the weekly grind sets in.